backbone.js

//     Underscore.js 1.3.3
//     (c) 2009-2012 Jeremy Ashkenas, DocumentCloud Inc.
//     Underscore is freely distributable under the MIT license.
//     Portions of Underscore are inspired or borrowed from Prototype,
//     Oliver Steele's Functional, and John Resig's Micro-Templating.
//     For all details and documentation:
//     http://documentcloud.github.com/underscore

(function() {

  // Baseline setup
  // --------------

  // Establish the root object, `window` in the browser, or `global` on the server.
  var root = this;

  // Save the previous value of the `_` variable.
  var previousUnderscore = root._;

  // Establish the object that gets returned to break out of a loop iteration.
  var breaker = {};

  // Save bytes in the minified (but not gzipped) version:
  var ArrayProto = Array.prototype, ObjProto = Object.prototype, FuncProto = Function.prototype;

  // Create quick reference variables for speed access to core prototypes.
  var slice            = ArrayProto.slice,
      unshift          = ArrayProto.unshift,
      toString         = ObjProto.toString,
      hasOwnProperty   = ObjProto.hasOwnProperty;

  // All **ECMAScript 5** native function implementations that we hope to use
  // are declared here.
  var
    nativeForEach      = ArrayProto.forEach,
    nativeMap          = ArrayProto.map,
    nativeReduce       = ArrayProto.reduce,
    nativeReduceRight  = ArrayProto.reduceRight,
    nativeFilter       = ArrayProto.filter,
    nativeEvery        = ArrayProto.every,
    nativeSome         = ArrayProto.some,
    nativeIndexOf      = ArrayProto.indexOf,
    nativeLastIndexOf  = ArrayProto.lastIndexOf,
    nativeIsArray      = Array.isArray,
    nativeKeys         = Object.keys,
    nativeBind         = FuncProto.bind;

  // Create a safe reference to the Underscore object for use below.
  var _ = function(obj) { return new wrapper(obj); };

  // Export the Underscore object for **Node.js**, with
  // backwards-compatibility for the old `require()` API. If we're in
  // the browser, add `_` as a global object via a string identifier,
  // for Closure Compiler "advanced" mode.
  if (typeof exports !== 'undefined') {
    if (typeof module !== 'undefined' && module.exports) {
      exports = module.exports = _;
    }
    exports._ = _;
  } else {
    root['_'] = _;
  }

  // Current version.
  _.VERSION = '1.3.3';

  // Collection Functions
  // --------------------

  // The cornerstone, an `each` implementation, aka `forEach`.
  // Handles objects with the built-in `forEach`, arrays, and raw objects.
  // Delegates to **ECMAScript 5**'s native `forEach` if available.
  var each = _.each = _.forEach = function(obj, iterator, context) {
    if (obj == null) return;
    if (nativeForEach && obj.forEach === nativeForEach) {
      obj.forEach(iterator, context);
    } else if (obj.length === +obj.length) {
      for (var i = 0, l = obj.length; i < l; i++) {
        if (i in obj && iterator.call(context, obj[i], i, obj) === breaker) return;
      }
    } else {
      for (var key in obj) {
        if (_.has(obj, key)) {
          if (iterator.call(context, obj[key], key, obj) === breaker) return;
        }
      }
    }
  };

  // Return the results of applying the iterator to each element.
  // Delegates to **ECMAScript 5**'s native `map` if available.
  _.map = _.collect = function(obj, iterator, context) {
    var results = [];
    if (obj == null) return results;
    if (nativeMap && obj.map === nativeMap) return obj.map(iterator, context);
    each(obj, function(value, index, list) {
      results[results.length] = iterator.call(context, value, index, list);
    });
    if (obj.length === +obj.length) results.length = obj.length;
    return results;
  };

  // **Reduce** builds up a single result from a list of values, aka `inject`,
  // or `foldl`. Delegates to **ECMAScript 5**'s native `reduce` if available.
  _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
    var initial = arguments.length > 2;
    if (obj == null) obj = [];
    if (nativeReduce && obj.reduce === nativeReduce) {
      if (context) iterator = _.bind(iterator, context);
      return initial ? obj.reduce(iterator, memo) : obj.reduce(iterator);
    }
    each(obj, function(value, index, list) {
      if (!initial) {
        memo = value;
        initial = true;
      } else {
        memo = iterator.call(context, memo, value, index, list);
      }
    });
    if (!initial) throw new TypeError('Reduce of empty array with no initial value');
    return memo;
  };

  // The right-associative version of reduce, also known as `foldr`.
  // Delegates to **ECMAScript 5**'s native `reduceRight` if available.
  _.reduceRight = _.foldr = function(obj, iterator, memo, context) {
    var initial = arguments.length > 2;
    if (obj == null) obj = [];
    if (nativeReduceRight && obj.reduceRight === nativeReduceRight) {
      if (context) iterator = _.bind(iterator, context);
      return initial ? obj.reduceRight(iterator, memo) : obj.reduceRight(iterator);
    }
    var reversed = _.toArray(obj).reverse();
    if (context && !initial) iterator = _.bind(iterator, context);
    return initial ? _.reduce(reversed, iterator, memo, context) : _.reduce(reversed, iterator);
  };

  // Return the first value which passes a truth test. Aliased as `detect`.
  _.find = _.detect = function(obj, iterator, context) {
    var result;
    any(obj, function(value, index, list) {
      if (iterator.call(context, value, index, list)) {
        result = value;
        return true;
      }
    });
    return result;
  };

  // Return all the elements that pass a truth test.
  // Delegates to **ECMAScript 5**'s native `filter` if available.
  // Aliased as `select`.
  _.filter = _.select = function(obj, iterator, context) {
    var results = [];
    if (obj == null) return results;
    if (nativeFilter && obj.filter === nativeFilter) return obj.filter(iterator, context);
    each(obj, function(value, index, list) {
      if (iterator.call(context, value, index, list)) results[results.length] = value;
    });
    return results;
  };

  // Return all the elements for which a truth test fails.
  _.reject = function(obj, iterator, context) {
    var results = [];
    if (obj == null) return results;
    each(obj, function(value, index, list) {
      if (!iterator.call(context, value, index, list)) results[results.length] = value;
    });
    return results;
  };

  // Determine whether all of the elements match a truth test.
  // Delegates to **ECMAScript 5**'s native `every` if available.
  // Aliased as `all`.
  _.every = _.all = function(obj, iterator, context) {
    var result = true;
    if (obj == null) return result;
    if (nativeEvery && obj.every === nativeEvery) return obj.every(iterator, context);
    each(obj, function(value, index, list) {
      if (!(result = result && iterator.call(context, value, index, list))) return breaker;
    });
    return !!result;
  };

  // Determine if at least one element in the object matches a truth test.
  // Delegates to **ECMAScript 5**'s native `some` if available.
  // Aliased as `any`.
  var any = _.some = _.any = function(obj, iterator, context) {
    iterator || (iterator = _.identity);
    var result = false;
    if (obj == null) return result;
    if (nativeSome && obj.some === nativeSome) return obj.some(iterator, context);
    each(obj, function(value, index, list) {
      if (result || (result = iterator.call(context, value, index, list))) return breaker;
    });
    return !!result;
  };

  // Determine if a given value is included in the array or object using `===`.
  // Aliased as `contains`.
  _.include = _.contains = function(obj, target) {
    var found = false;
    if (obj == null) return found;
    if (nativeIndexOf && obj.indexOf === nativeIndexOf) return obj.indexOf(target) != -1;
    found = any(obj, function(value) {
      return value === target;
    });
    return found;
  };

  // Invoke a method (with arguments) on every item in a collection.
  _.invoke = function(obj, method) {
    var args = slice.call(arguments, 2);
    return _.map(obj, function(value) {
      return (_.isFunction(method) ? method || value : value[method]).apply(value, args);
    });
  };

  // Convenience version of a common use case of `map`: fetching a property.
  _.pluck = function(obj, key) {
    return _.map(obj, function(value){ return value[key]; });
  };

  // Return the maximum element or (element-based computation).
  _.max = function(obj, iterator, context) {
    if (!iterator && _.isArray(obj) && obj[0] === +obj[0]) return Math.max.apply(Math, obj);
    if (!iterator && _.isEmpty(obj)) return -Infinity;
    var result = {computed : -Infinity};
    each(obj, function(value, index, list) {
      var computed = iterator ? iterator.call(context, value, index, list) : value;
      computed >= result.computed && (result = {value : value, computed : computed});
    });
    return result.value;
  };

  // Return the minimum element (or element-based computation).
  _.min = function(obj, iterator, context) {
    if (!iterator && _.isArray(obj) && obj[0] === +obj[0]) return Math.min.apply(Math, obj);
    if (!iterator && _.isEmpty(obj)) return Infinity;
    var result = {computed : Infinity};
    each(obj, function(value, index, list) {
      var computed = iterator ? iterator.call(context, value, index, list) : value;
      computed < result.computed && (result = {value : value, computed : computed});
    });
    return result.value;
  };

  // Shuffle an array.
  _.shuffle = function(obj) {
    var shuffled = [], rand;
    each(obj, function(value, index, list) {
      rand = Math.floor(Math.random() * (index + 1));
      shuffled[index] = shuffled[rand];
      shuffled[rand] = value;
    });
    return shuffled;
  };

  // Sort the object's values by a criterion produced by an iterator.
  _.sortBy = function(obj, val, context) {
    var iterator = _.isFunction(val) ? val : function(obj) { return obj[val]; };
    return _.pluck(_.map(obj, function(value, index, list) {
      return {
        value : value,
        criteria : iterator.call(context, value, index, list)
      };
    }).sort(function(left, right) {
      var a = left.criteria, b = right.criteria;
      if (a === void 0) return 1;
      if (b === void 0) return -1;
      return a < b ? -1 : a > b ? 1 : 0;
    }), 'value');
  };

  // Groups the object's values by a criterion. Pass either a string attribute
  // to group by, or a function that returns the criterion.
  _.groupBy = function(obj, val) {
    var result = {};
    var iterator = _.isFunction(val) ? val : function(obj) { return obj[val]; };
    each(obj, function(value, index) {
      var key = iterator(value, index);
      (result[key] || (result[key] = [])).push(value);
    });
    return result;
  };

  // Use a comparator function to figure out at what index an object should
  // be inserted so as to maintain order. Uses binary search.
  _.sortedIndex = function(array, obj, iterator) {
    iterator || (iterator = _.identity);
    var low = 0, high = array.length;
    while (low < high) {
      var mid = (low + high) >> 1;
      iterator(array[mid]) < iterator(obj) ? low = mid + 1 : high = mid;
    }
    return low;
  };

  // Safely convert anything iterable into a real, live array.
  _.toArray = function(obj) {
    if (!obj)                                     return [];
    if (_.isArray(obj))                           return slice.call(obj);
    if (_.isArguments(obj))                       return slice.call(obj);
    if (obj.toArray && _.isFunction(obj.toArray)) return obj.toArray();
    return _.values(obj);
  };

  // Return the number of elements in an object.
  _.size = function(obj) {
    return _.isArray(obj) ? obj.length : _.keys(obj).length;
  };

  // Array Functions
  // ---------------

  // Get the first element of an array. Passing **n** will return the first N
  // values in the array. Aliased as `head` and `take`. The **guard** check
  // allows it to work with `_.map`.
  _.first = _.head = _.take = function(array, n, guard) {
    return (n != null) && !guard ? slice.call(array, 0, n) : array[0];
  };

  // Returns everything but the last entry of the array. Especcialy useful on
  // the arguments object. Passing **n** will return all the values in
  // the array, excluding the last N. The **guard** check allows it to work with
  // `_.map`.
  _.initial = function(array, n, guard) {
    return slice.call(array, 0, array.length - ((n == null) || guard ? 1 : n));
  };

  // Get the last element of an array. Passing **n** will return the last N
  // values in the array. The **guard** check allows it to work with `_.map`.
  _.last = function(array, n, guard) {
    if ((n != null) && !guard) {
      return slice.call(array, Math.max(array.length - n, 0));
    } else {
      return array[array.length - 1];
    }
  };

  // Returns everything but the first entry of the array. Aliased as `tail`.
  // Especially useful on the arguments object. Passing an **index** will return
  // the rest of the values in the array from that index onward. The **guard**
  // check allows it to work with `_.map`.
  _.rest = _.tail = function(array, index, guard) {
    return slice.call(array, (index == null) || guard ? 1 : index);
  };

  // Trim out all falsy values from an array.
  _.compact = function(array) {
    return _.filter(array, function(value){ return !!value; });
  };

  // Return a completely flattened version of an array.
  _.flatten = function(array, shallow) {
    return _.reduce(array, function(memo, value) {
      if (_.isArray(value)) return memo.concat(shallow ? value : _.flatten(value));
      memo[memo.length] = value;
      return memo;
    }, []);
  };

  // Return a version of the array that does not contain the specified value(s).
  _.without = function(array) {
    return _.difference(array, slice.call(arguments, 1));
  };

  // Produce a duplicate-free version of the array. If the array has already
  // been sorted, you have the option of using a faster algorithm.
  // Aliased as `unique`.
  _.uniq = _.unique = function(array, isSorted, iterator) {
    var initial = iterator ? _.map(array, iterator) : array;
    var results = [];
    // The `isSorted` flag is irrelevant if the array only contains two elements.
    if (array.length < 3) isSorted = true;
    _.reduce(initial, function (memo, value, index) {
      if (isSorted ? _.last(memo) !== value || !memo.length : !_.include(memo, value)) {
        memo.push(value);
        results.push(array[index]);
      }
      return memo;
    }, []);
    return results;
  };

  // Produce an array that contains the union: each distinct element from all of
  // the passed-in arrays.
  _.union = function() {
    return _.uniq(_.flatten(arguments, true));
  };

  // Produce an array that contains every item shared between all the
  // passed-in arrays. (Aliased as "intersect" for back-compat.)
  _.intersection = _.intersect = function(array) {
    var rest = slice.call(arguments, 1);
    return _.filter(_.uniq(array), function(item) {
      return _.every(rest, function(other) {
        return _.indexOf(other, item) >= 0;
      });
    });
  };

  // Take the difference between one array and a number of other arrays.
  // Only the elements present in just the first array will remain.
  _.difference = function(array) {
    var rest = _.flatten(slice.call(arguments, 1), true);
    return _.filter(array, function(value){ return !_.include(rest, value); });
  };

  // Zip together multiple lists into a single array -- elements that share
  // an index go together.
  _.zip = function() {
    var args = slice.call(arguments);
    var length = _.max(_.pluck(args, 'length'));
    var results = new Array(length);
    for (var i = 0; i < length; i++) results[i] = _.pluck(args, "" + i);
    return results;
  };

  // If the browser doesn't supply us with indexOf (I'm looking at you, **MSIE**),
  // we need this function. Return the position of the first occurrence of an
  // item in an array, or -1 if the item is not included in the array.
  // Delegates to **ECMAScript 5**'s native `indexOf` if available.
  // If the array is large and already in sort order, pass `true`
  // for **isSorted** to use binary search.
  _.indexOf = function(array, item, isSorted) {
    if (array == null) return -1;
    var i, l;
    if (isSorted) {
      i = _.sortedIndex(array, item);
      return array[i] === item ? i : -1;
    }
    if (nativeIndexOf && array.indexOf === nativeIndexOf) return array.indexOf(item);
    for (i = 0, l = array.length; i < l; i++) if (i in array && array[i] === item) return i;
    return -1;
  };

  // Delegates to **ECMAScript 5**'s native `lastIndexOf` if available.
  _.lastIndexOf = function(array, item) {
    if (array == null) return -1;
    if (nativeLastIndexOf && array.lastIndexOf === nativeLastIndexOf) return array.lastIndexOf(item);
    var i = array.length;
    while (i--) if (i in array && array[i] === item) return i;
    return -1;
  };

  // Generate an integer Array containing an arithmetic progression. A port of
  // the native Python `range()` function. See
  // [the Python documentation](http://docs.python.org/library/functions.html#range).
  _.range = function(start, stop, step) {
    if (arguments.length <= 1) {
      stop = start || 0;
      start = 0;
    }
    step = arguments[2] || 1;

    var len = Math.max(Math.ceil((stop - start) / step), 0);
    var idx = 0;
    var range = new Array(len);

    while(idx < len) {
      range[idx++] = start;
      start += step;
    }

    return range;
  };

  // Function (ahem) Functions
  // ------------------

  // Reusable constructor function for prototype setting.
  var ctor = function(){};

  // Create a function bound to a given object (assigning `this`, and arguments,
  // optionally). Binding with arguments is also known as `curry`.
  // Delegates to **ECMAScript 5**'s native `Function.bind` if available.
  // We check for `func.bind` first, to fail fast when `func` is undefined.
  _.bind = function bind(func, context) {
    var bound, args;
    if (func.bind === nativeBind && nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
    if (!_.isFunction(func)) throw new TypeError;
    args = slice.call(arguments, 2);
    return bound = function() {
      if (!(this instanceof bound)) return func.apply(context, args.concat(slice.call(arguments)));
      ctor.prototype = func.prototype;
      var self = new ctor;
      var result = func.apply(self, args.concat(slice.call(arguments)));
      if (Object(result) === result) return result;
      return self;
    };
  };

  // Bind all of an object's methods to that object. Useful for ensuring that
  // all callbacks defined on an object belong to it.
  _.bindAll = function(obj) {
    var funcs = slice.call(arguments, 1);
    if (funcs.length == 0) funcs = _.functions(obj);
    each(funcs, function(f) { obj[f] = _.bind(obj[f], obj); });
    return obj;
  };

  // Memoize an expensive function by storing its results.
  _.memoize = function(func, hasher) {
    var memo = {};
    hasher || (hasher = _.identity);
    return function() {
      var key = hasher.apply(this, arguments);
      return _.has(memo, key) ? memo[key] : (memo[key] = func.apply(this, arguments));
    };
  };

  // Delays a function for the given number of milliseconds, and then calls
  // it with the arguments supplied.
  _.delay = function(func, wait) {
    var args = slice.call(arguments, 2);
    return setTimeout(function(){ return func.apply(null, args); }, wait);
  };

  // Defers a function, scheduling it to run after the current call stack has
  // cleared.
  _.defer = function(func) {
    return _.delay.apply(_, [func, 1].concat(slice.call(arguments, 1)));
  };

  // Returns a function, that, when invoked, will only be triggered at most once
  // during a given window of time.
  _.throttle = function(func, wait) {
    var context, args, timeout, throttling, more, result;
    var whenDone = _.debounce(function(){ more = throttling = false; }, wait);
    return function() {
      context = this; args = arguments;
      var later = function() {
        timeout = null;
        if (more) func.apply(context, args);
        whenDone();
      };
      if (!timeout) timeout = setTimeout(later, wait);
      if (throttling) {
        more = true;
      } else {
        result = func.apply(context, args);
      }
      whenDone();
      throttling = true;
      return result;
    };
  };

  // Returns a function, that, as long as it continues to be invoked, will not
  // be triggered. The function will be called after it stops being called for
  // N milliseconds. If `immediate` is passed, trigger the function on the
  // leading edge, instead of the trailing.
  _.debounce = function(func, wait, immediate) {
    var timeout;
    return function() {
      var context = this, args = arguments;
      var later = function() {
        timeout = null;
        if (!immediate) func.apply(context, args);
      };
      if (immediate && !timeout) func.apply(context, args);
      clearTimeout(timeout);
      timeout = setTimeout(later, wait);
    };
  };

  // Returns a function that will be executed at most one time, no matter how
  // often you call it. Useful for lazy initialization.
  _.once = function(func) {
    var ran = false, memo;
    return function() {
      if (ran) return memo;
      ran = true;
      return memo = func.apply(this, arguments);
    };
  };

  // Returns the first function passed as an argument to the second,
  // allowing you to adjust arguments, run code before and after, and
  // conditionally execute the original function.
  _.wrap = function(func, wrapper) {
    return function() {
      var args = [func].concat(slice.call(arguments, 0));
      return wrapper.apply(this, args);
    };
  };

  // Returns a function that is the composition of a list of functions, each
  // consuming the return value of the function that follows.
  _.compose = function() {
    var funcs = arguments;
    return function() {
      var args = arguments;
      for (var i = funcs.length - 1; i >= 0; i--) {
        args = [funcs[i].apply(this, args)];
      }
      return args[0];
    };
  };

  // Returns a function that will only be executed after being called N times.
  _.after = function(times, func) {
    if (times <= 0) return func();
    return function() {
      if (--times < 1) { return func.apply(this, arguments); }
    };
  };

  // Object Functions
  // ----------------

  // Retrieve the names of an object's properties.
  // Delegates to **ECMAScript 5**'s native `Object.keys`
  _.keys = nativeKeys || function(obj) {
    if (obj !== Object(obj)) throw new TypeError('Invalid object');
    var keys = [];
    for (var key in obj) if (_.has(obj, key)) keys[keys.length] = key;
    return keys;
  };

  // Retrieve the values of an object's properties.
  _.values = function(obj) {
    return _.map(obj, _.identity);
  };

  // Return a sorted list of the function names available on the object.
  // Aliased as `methods`
  _.functions = _.methods = function(obj) {
    var names = [];
    for (var key in obj) {
      if (_.isFunction(obj[key])) names.push(key);
    }
    return names.sort();
  };

  // Extend a given object with all the properties in passed-in object(s).
  _.extend = function(obj) {
    each(slice.call(arguments, 1), function(source) {
      for (var prop in source) {
        obj[prop] = source[prop];
      }
    });
    return obj;
  };

  // Return a copy of the object only containing the whitelisted properties.
  _.pick = function(obj) {
    var result = {};
    each(_.flatten(slice.call(arguments, 1)), function(key) {
      if (key in obj) result[key] = obj[key];
    });
    return result;
  };

  // Fill in a given object with default properties.
  _.defaults = function(obj) {
    each(slice.call(arguments, 1), function(source) {
      for (var prop in source) {
        if (obj[prop] == null) obj[prop] = source[prop];
      }
    });
    return obj;
  };

  // Create a (shallow-cloned) duplicate of an object.
  _.clone = function(obj) {
    if (!_.isObject(obj)) return obj;
    return _.isArray(obj) ? obj.slice() : _.extend({}, obj);
  };

  // Invokes interceptor with the obj, and then returns obj.
  // The primary purpose of this method is to "tap into" a method chain, in
  // order to perform operations on intermediate results within the chain.
  _.tap = function(obj, interceptor) {
    interceptor(obj);
    return obj;
  };

  // Internal recursive comparison function.
  function eq(a, b, stack) {
    // Identical objects are equal. `0 === -0`, but they aren't identical.
    // See the Harmony `egal` proposal: http://wiki.ecmascript.org/doku.php?id=harmony:egal.
    if (a === b) return a !== 0 || 1 / a == 1 / b;
    // A strict comparison is necessary because `null == undefined`.
    if (a == null || b == null) return a === b;
    // Unwrap any wrapped objects.
    if (a._chain) a = a._wrapped;
    if (b._chain) b = b._wrapped;
    // Invoke a custom `isEqual` method if one is provided.
    if (a.isEqual && _.isFunction(a.isEqual)) return a.isEqual(b);
    if (b.isEqual && _.isFunction(b.isEqual)) return b.isEqual(a);
    // Compare `[[Class]]` names.
    var className = toString.call(a);
    if (className != toString.call(b)) return false;
    switch (className) {
      // Strings, numbers, dates, and booleans are compared by value.
      case '[object String]':
        // Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is
        // equivalent to `new String("5")`.
        return a == String(b);
      case '[object Number]':
        // `NaN`s are equivalent, but non-reflexive. An `egal` comparison is performed for
        // other numeric values.
        return a != +a ? b != +b : (a == 0 ? 1 / a == 1 / b : a == +b);
      case '[object Date]':
      case '[object Boolean]':
        // Coerce dates and booleans to numeric primitive values. Dates are compared by their
        // millisecond representations. Note that invalid dates with millisecond representations
        // of `NaN` are not equivalent.
        return +a == +b;
      // RegExps are compared by their source patterns and flags.
      case '[object RegExp]':
        return a.source == b.source &&
               a.global == b.global &&
               a.multiline == b.multiline &&
               a.ignoreCase == b.ignoreCase;
    }
    if (typeof a != 'object' || typeof b != 'object') return false;
    // Assume equality for cyclic structures. The algorithm for detecting cyclic
    // structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`.
    var length = stack.length;
    while (length--) {
      // Linear search. Performance is inversely proportional to the number of
      // unique nested structures.
      if (stack[length] == a) return true;
    }
    // Add the first object to the stack of traversed objects.
    stack.push(a);
    var size = 0, result = true;
    // Recursively compare objects and arrays.
    if (className == '[object Array]') {
      // Compare array lengths to determine if a deep comparison is necessary.
      size = a.length;
      result = size == b.length;
      if (result) {
        // Deep compare the contents, ignoring non-numeric properties.
        while (size--) {
          // Ensure commutative equality for sparse arrays.
          if (!(result = size in a == size in b && eq(a[size], b[size], stack))) break;
        }
      }
    } else {
      // Objects with different constructors are not equivalent.
      if ('constructor' in a != 'constructor' in b || a.constructor != b.constructor) return false;
      // Deep compare objects.
      for (var key in a) {
        if (_.has(a, key)) {
          // Count the expected number of properties.
          size++;
          // Deep compare each member.
          if (!(result = _.has(b, key) && eq(a[key], b[key], stack))) break;
        }
      }
      // Ensure that both objects contain the same number of properties.
      if (result) {
        for (key in b) {
          if (_.has(b, key) && !(size--)) break;
        }
        result = !size;
      }
    }
    // Remove the first object from the stack of traversed objects.
    stack.pop();
    return result;
  }

  // Perform a deep comparison to check if two objects are equal.
  _.isEqual = function(a, b) {
    return eq(a, b, []);
  };

  // Is a given array, string, or object empty?
  // An "empty" object has no enumerable own-properties.
  _.isEmpty = function(obj) {
    if (obj == null) return true;
    if (_.isArray(obj) || _.isString(obj)) return obj.length === 0;
    for (var key in obj) if (_.has(obj, key)) return false;
    return true;
  };

  // Is a given value a DOM element?
  _.isElement = function(obj) {
    return !!(obj && obj.nodeType == 1);
  };

  // Is a given value an array?
  // Delegates to ECMA5's native Array.isArray
  _.isArray = nativeIsArray || function(obj) {
    return toString.call(obj) == '[object Array]';
  };

  // Is a given variable an object?
  _.isObject = function(obj) {
    return obj === Object(obj);
  };

  // Is a given variable an arguments object?
  _.isArguments = function(obj) {
    return toString.call(obj) == '[object Arguments]';
  };
  if (!_.isArguments(arguments)) {
    _.isArguments = function(obj) {
      return !!(obj && _.has(obj, 'callee'));
    };
  }

  // Is a given value a function?
  _.isFunction = function(obj) {
    return toString.call(obj) == '[object Function]';
  };

  // Is a given value a string?
  _.isString = function(obj) {
    return toString.call(obj) == '[object String]';
  };

  // Is a given value a number?
  _.isNumber = function(obj) {
    return toString.call(obj) == '[object Number]';
  };

  // Is a given object a finite number?
  _.isFinite = function(obj) {
    return _.isNumber(obj) && isFinite(obj);
  };

  // Is the given value `NaN`?
  _.isNaN = function(obj) {
    // `NaN` is the only value for which `===` is not reflexive.
    return obj !== obj;
  };

  // Is a given value a boolean?
  _.isBoolean = function(obj) {
    return obj === true || obj === false || toString.call(obj) == '[object Boolean]';
  };

  // Is a given value a date?
  _.isDate = function(obj) {
    return toString.call(obj) == '[object Date]';
  };

  // Is the given value a regular expression?
  _.isRegExp = function(obj) {
    return toString.call(obj) == '[object RegExp]';
  };

  // Is a given value equal to null?
  _.isNull = function(obj) {
    return obj === null;
  };

  // Is a given variable undefined?
  _.isUndefined = function(obj) {
    return obj === void 0;
  };

  // Has own property?
  _.has = function(obj, key) {
    return hasOwnProperty.call(obj, key);
  };

  // Utility Functions
  // -----------------

  // Run Underscore.js in *noConflict* mode, returning the `_` variable to its
  // previous owner. Returns a reference to the Underscore object.
  _.noConflict = function() {
    root._ = previousUnderscore;
    return this;
  };

  // Keep the identity function around for default iterators.
  _.identity = function(value) {
    return value;
  };

  // Run a function **n** times.
  _.times = function (n, iterator, context) {
    for (var i = 0; i < n; i++) iterator.call(context, i);
  };

  // Escape a string for HTML interpolation.
  _.escape = function(string) {
    return (''+string).replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;').replace(/'/g, '&#x27;').replace(/\//g,'&#x2F;');
  };

  // If the value of the named property is a function then invoke it;
  // otherwise, return it.
  _.result = function(object, property) {
    if (object == null) return null;
    var value = object[property];
    return _.isFunction(value) ? value.call(object) : value;
  };

  // Add your own custom functions to the Underscore object, ensuring that
  // they're correctly added to the OOP wrapper as well.
  _.mixin = function(obj) {
    each(_.functions(obj), function(name){
      addToWrapper(name, _[name] = obj[name]);
    });
  };

  // Generate a unique integer id (unique within the entire client session).
  // Useful for temporary DOM ids.
  var idCounter = 0;
  _.uniqueId = function(prefix) {
    var id = idCounter++;
    return prefix ? prefix + id : id;
  };

  // By default, Underscore uses ERB-style template delimiters, change the
  // following template settings to use alternative delimiters.
  _.templateSettings = {
    evaluate    : /<%([\s\S]+?)%>/g,
    interpolate : /<%=([\s\S]+?)%>/g,
    escape      : /<%-([\s\S]+?)%>/g
  };

  // When customizing `templateSettings`, if you don't want to define an
  // interpolation, evaluation or escaping regex, we need one that is
  // guaranteed not to match.
  var noMatch = /.^/;

  // Certain characters need to be escaped so that they can be put into a
  // string literal.
  var escapes = {
    '\\': '\\',
    "'": "'",
    'r': '\r',
    'n': '\n',
    't': '\t',
    'u2028': '\u2028',
    'u2029': '\u2029'
  };

  for (var p in escapes) escapes[escapes[p]] = p;
  var escaper = /\\|'|\r|\n|\t|\u2028|\u2029/g;
  var unescaper = /\\(\\|'|r|n|t|u2028|u2029)/g;

  // Within an interpolation, evaluation, or escaping, remove HTML escaping
  // that had been previously added.
  var unescape = function(code) {
    return code.replace(unescaper, function(match, escape) {
      return escapes[escape];
    });
  };

  // JavaScript micro-templating, similar to John Resig's implementation.
  // Underscore templating handles arbitrary delimiters, preserves whitespace,
  // and correctly escapes quotes within interpolated code.
  _.template = function(text, data, settings) {
    settings = _.defaults(settings || {}, _.templateSettings);

    // Compile the template source, taking care to escape characters that
    // cannot be included in a string literal and then unescape them in code
    // blocks.
    var source = "__p+='" + text
      .replace(escaper, function(match) {
        return '\\' + escapes[match];
      })
      .replace(settings.escape || noMatch, function(match, code) {
        return "'+\n_.escape(" + unescape(code) + ")+\n'";
      })
      .replace(settings.interpolate || noMatch, function(match, code) {
        return "'+\n(" + unescape(code) + ")+\n'";
      })
      .replace(settings.evaluate || noMatch, function(match, code) {
        return "';\n" + unescape(code) + "\n;__p+='";
      }) + "';\n";

    // If a variable is not specified, place data values in local scope.
    if (!settings.variable) source = 'with(obj||{}){\n' + source + '}\n';

    source = "var __p='';" +
      "var print=function(){__p+=Array.prototype.join.call(arguments, '')};\n" +
      source + "return __p;\n";

    var render = new Function(settings.variable || 'obj', '_', source);
    if (data) return render(data, _);
    var template = function(data) {
      return render.call(this, data, _);
    };

    // Provide the compiled function source as a convenience for build time
    // precompilation.
    template.source = 'function(' + (settings.variable || 'obj') + '){\n' +
      source + '}';

    return template;
  };

  // Add a "chain" function, which will delegate to the wrapper.
  _.chain = function(obj) {
    return _(obj).chain();
  };

  // The OOP Wrapper
  // ---------------

  // If Underscore is called as a function, it returns a wrapped object that
  // can be used OO-style. This wrapper holds altered versions of all the
  // underscore functions. Wrapped objects may be chained.
  var wrapper = function(obj) { this._wrapped = obj; };

  // Expose `wrapper.prototype` as `_.prototype`
  _.prototype = wrapper.prototype;

  // Helper function to continue chaining intermediate results.
  var result = function(obj, chain) {
    return chain ? _(obj).chain() : obj;
  };

  // A method to easily add functions to the OOP wrapper.
  var addToWrapper = function(name, func) {
    wrapper.prototype[name] = function() {
      var args = slice.call(arguments);
      unshift.call(args, this._wrapped);
      return result(func.apply(_, args), this._chain);
    };
  };

  // Add all of the Underscore functions to the wrapper object.
  _.mixin(_);

  // Add all mutator Array functions to the wrapper.
  each(['pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift'], function(name) {
    var method = ArrayProto[name];
    wrapper.prototype[name] = function() {
      var wrapped = this._wrapped;
      method.apply(wrapped, arguments);
      var length = wrapped.length;
      if ((name == 'shift' || name == 'splice') && length === 0) delete wrapped[0];
      return result(wrapped, this._chain);
    };
  });

  // Add all accessor Array functions to the wrapper.
  each(['concat', 'join', 'slice'], function(name) {
    var method = ArrayProto[name];
    wrapper.prototype[name] = function() {
      return result(method.apply(this._wrapped, arguments), this._chain);
    };
  });

  // Start chaining a wrapped Underscore object.
  wrapper.prototype.chain = function() {
    this._chain = true;
    return this;
  };

  // Extracts the result from a wrapped and chained object.
  wrapper.prototype.value = function() {
    return this._wrapped;
  };

}).call(this);

//     Backbone.js 0.9.2

//     (c) 2010-2012 Jeremy Ashkenas, DocumentCloud Inc.
//     Backbone may be freely distributed under the MIT license.
//     For all details and documentation:
//     http://backbonejs.org

(function(){

  // Initial Setup
  // -------------

  // Save a reference to the global object (`window` in the browser, `global`
  // on the server).
  var root = this;

  // Save the previous value of the `Backbone` variable, so that it can be
  // restored later on, if `noConflict` is used.
  var previousBackbone = root.Backbone;

  // Create a local reference to slice/splice.
  var slice = Array.prototype.slice;
  var splice = Array.prototype.splice;

  // The top-level namespace. All public Backbone classes and modules will
  // be attached to this. Exported for both CommonJS and the browser.
  var Backbone;
  if (typeof exports !== 'undefined') {
    Backbone = exports;
  } else {
    Backbone = root.Backbone = {};
  }

  // Current version of the library. Keep in sync with `package.json`.
  Backbone.VERSION = '0.9.2';

  // Require Underscore, if we're on the server, and it's not already present.
  var _ = root._;
  if (!_ && (typeof require !== 'undefined')) _ = require('underscore');

  // For Backbone's purposes, jQuery, Zepto, or Ender owns the `$` variable.
  var $ = root.jQuery || root.Zepto || root.ender;

  // Set the JavaScript library that will be used for DOM manipulation and
  // Ajax calls (a.k.a. the `$` variable). By default Backbone will use: jQuery,
  // Zepto, or Ender; but the `setDomLibrary()` method lets you inject an
  // alternate JavaScript library (or a mock library for testing your views
  // outside of a browser).
  Backbone.setDomLibrary = function(lib) {
    $ = lib;
  };

  // Runs Backbone.js in *noConflict* mode, returning the `Backbone` variable
  // to its previous owner. Returns a reference to this Backbone object.
  Backbone.noConflict = function() {
    root.Backbone = previousBackbone;
    return this;
  };

  // Turn on `emulateHTTP` to support legacy HTTP servers. Setting this option
  // will fake `"PUT"` and `"DELETE"` requests via the `_method` parameter and
  // set a `X-Http-Method-Override` header.
  Backbone.emulateHTTP = false;

  // Turn on `emulateJSON` to support legacy servers that can't deal with direct
  // `application/json` requests ... will encode the body as
  // `application/x-www-form-urlencoded` instead and will send the model in a
  // form param named `model`.
  Backbone.emulateJSON = false;

  // Backbone.Events
  // -----------------

  // Regular expression used to split event strings
  var eventSplitter = /\s+/;

  // A module that can be mixed in to *any object* in order to provide it with
  // custom events. You may bind with `on` or remove with `off` callback functions
  // to an event; trigger`-ing an event fires all callbacks in succession.
  //
  //     var object = {};
  //     _.extend(object, Backbone.Events);
  //     object.on('expand', function(){ alert('expanded'); });
  //     object.trigger('expand');
  //
  var Events = Backbone.Events = {

    // Bind one or more space separated events, `events`, to a `callback`
    // function. Passing `"all"` will bind the callback to all events fired.
    on: function(events, callback, context) {

      var calls, event, node, tail, list;
      if (!callback) return this;
      events = events.split(eventSplitter);
      calls = this._callbacks || (this._callbacks = {});

      // Create an immutable callback list, allowing traversal during
      // modification.  The tail is an empty object that will always be used
      // as the next node.
      while (event = events.shift()) {
        list = calls[event];
        node = list ? list.tail : {};
        node.next = tail = {};
        node.context = context;
        node.callback = callback;
        calls[event] = {tail: tail, next: list ? list.next : node};
      }

      return this;
    },

    // Remove one or many callbacks. If `context` is null, removes all callbacks
    // with that function. If `callback` is null, removes all callbacks for the
    // event. If `events` is null, removes all bound callbacks for all events.
    off: function(events, callback, context) {
      var event, calls, node, tail, cb, ctx;

      // No events, or removing *all* events.
      if (!(calls = this._callbacks)) return;
      if (!(events || callback || context)) {
        delete this._callbacks;
        return this;
      }

      // Loop through the listed events and contexts, splicing them out of the
      // linked list of callbacks if appropriate.
      events = events ? events.split(eventSplitter) : _.keys(calls);
      while (event = events.shift()) {
        node = calls[event];
        delete calls[event];
        if (!node || !(callback || context)) continue;
        // Create a new list, omitting the indicated callbacks.
        tail = node.tail;
        while ((node = node.next) !== tail) {
          cb = node.callback;
          ctx = node.context;
          if ((callback && cb !== callback) || (context && ctx !== context)) {
            this.on(event, cb, ctx);
          }
        }
      }

      return this;
    },

    // Trigger one or many events, firing all bound callbacks. Callbacks are
    // passed the same arguments as `trigger` is, apart from the event name
    // (unless you're listening on `"all"`, which will cause your callback to
    // receive the true name of the event as the first argument).
    trigger: function(events) {
      var event, node, calls, tail, args, all, rest;
      if (!(calls = this._callbacks)) return this;
      all = calls.all;
      events = events.split(eventSplitter);
      rest = slice.call(arguments, 1);

      // For each event, walk through the linked list of callbacks twice,
      // first to trigger the event, then to trigger any `"all"` callbacks.
      while (event = events.shift()) {
        if (node = calls[event]) {
          tail = node.tail;
          while ((node = node.next) !== tail) {
            node.callback.apply(node.context || this, rest);
          }
        }
        if (node = all) {
          tail = node.tail;
          args = [event].concat(rest);
          while ((node = node.next) !== tail) {
            node.callback.apply(node.context || this, args);
          }
        }
      }

      return this;
    }

  };

  // Aliases for backwards compatibility.
  Events.bind   = Events.on;
  Events.unbind = Events.off;

  // Backbone.Model
  // --------------

  // Create a new model, with defined attributes. A client id (`cid`)
  // is automatically generated and assigned for you.
  var Model = Backbone.Model = function(attributes, options) {
    var defaults;
    attributes || (attributes = {});
    if (options && options.parse) attributes = this.parse(attributes);
    if (defaults = getValue(this, 'defaults')) {
      attributes = _.extend({}, defaults, attributes);
    }
    if (options && options.collection) this.collection = options.collection;
    this.attributes = {};
    this._escapedAttributes = {};
    this.cid = _.uniqueId('c');
    this.changed = {};
    this._silent = {};
    this._pending = {};
    this.set(attributes, {silent: true});
    // Reset change tracking.
    this.changed = {};
    this._silent = {};
    this._pending = {};
    this._previousAttributes = _.clone(this.attributes);
    this.initialize.apply(this, arguments);
  };

  // Attach all inheritable methods to the Model prototype.
  _.extend(Model.prototype, Events, {

    // A hash of attributes whose current and previous value differ.
    changed: null,

    // A hash of attributes that have silently changed since the last time
    // `change` was called.  Will become pending attributes on the next call.
    _silent: null,

    // A hash of attributes that have changed since the last `'change'` event
    // began.
    _pending: null,

    // The default name for the JSON `id` attribute is `"id"`. MongoDB and
    // CouchDB users may want to set this to `"_id"`.
    idAttribute: 'id',

    // Initialize is an empty function by default. Override it with your own
    // initialization logic.
    initialize: function(){},

    // Return a copy of the model's `attributes` object.
    toJSON: function(options) {
      return _.clone(this.attributes);
    },

    // Get the value of an attribute.
    get: function(attr) {
      return this.attributes[attr];
    },

    // Get the HTML-escaped value of an attribute.
    escape: function(attr) {
      var html;
      if (html = this._escapedAttributes[attr]) return html;
      var val = this.get(attr);
      return this._escapedAttributes[attr] = _.escape(val == null ? '' : '' + val);
    },

    // Returns `true` if the attribute contains a value that is not null
    // or undefined.
    has: function(attr) {
      return this.get(attr) != null;
    },

    // Set a hash of model attributes on the object, firing `"change"` unless
    // you choose to silence it.
    set: function(key, value, options) {
      var attrs, attr, val;

      // Handle both `"key", value` and `{key: value}` -style arguments.
      if (_.isObject(key) || key == null) {
        attrs = key;
        options = value;
      } else {
        attrs = {};
        attrs[key] = value;
      }

      // Extract attributes and options.
      options || (options = {});
      if (!attrs) return this;
      if (attrs instanceof Model) attrs = attrs.attributes;
      if (options.unset) for (attr in attrs) attrs[attr] = void 0;

      // Run validation.
      if (!this._validate(attrs, options)) return false;

      // Check for changes of `id`.
      if (this.idAttribute in attrs) this.id = attrs[this.idAttribute];

      var changes = options.changes = {};
      var now = this.attributes;
      var escaped = this._escapedAttributes;
      var prev = this._previousAttributes || {};

      // For each `set` attribute...
      for (attr in attrs) {
        val = attrs[attr];

        // If the new and current value differ, record the change.
        if (!_.isEqual(now[attr], val) || (options.unset && _.has(now, attr))) {
          delete escaped[attr];
          (options.silent ? this._silent : changes)[attr] = true;
        }

        // Update or delete the current value.
        options.unset ? delete now[attr] : now[attr] = val;

        // If the new and previous value differ, record the change.  If not,
        // then remove changes for this attribute.
        if (!_.isEqual(prev[attr], val) || (_.has(now, attr) != _.has(prev, attr))) {
          this.changed[attr] = val;
          if (!options.silent) this._pending[attr] = true;
        } else {
          delete this.changed[attr];
          delete this._pending[attr];
        }
      }

      // Fire the `"change"` events.
      if (!options.silent) this.change(options);
      return this;
    },

    // Remove an attribute from the model, firing `"change"` unless you choose
    // to silence it. `unset` is a noop if the attribute doesn't exist.
    unset: function(attr, options) {
      (options || (options = {})).unset = true;
      return this.set(attr, null, options);
    },

    // Clear all attributes on the model, firing `"change"` unless you choose
    // to silence it.
    clear: function(options) {
      (options || (options = {})).unset = true;
      return this.set(_.clone(this.attributes), options);
    },

    // Fetch the model from the server. If the server's representation of the
    // model differs from its current attributes, they will be overriden,
    // triggering a `"change"` event.
    fetch: function(options) {
      options = options ? _.clone(options) : {};
      var model = this;
      var success = options.success;
      options.success = function(resp, status, xhr) {
        if (!model.set(model.parse(resp, xhr), options)) return false;
        if (success) success(model, resp);
      };
      options.error = Backbone.wrapError(options.error, model, options);
      return (this.sync || Backbone.sync).call(this, 'read', this, options);
    },

    // Set a hash of model attributes, and sync the model to the server.
    // If the server returns an attributes hash that differs, the model's
    // state will be `set` again.
    save: function(key, value, options) {
      var attrs, current;

      // Handle both `("key", value)` and `({key: value})` -style calls.
      if (_.isObject(key) || key == null) {
        attrs = key;
        options = value;
      } else {
        attrs = {};
        attrs[key] = value;
      }
      options = options ? _.clone(options) : {};

      // If we're "wait"-ing to set changed attributes, validate early.
      if (options.wait) {
        if (!this._validate(attrs, options)) return false;
        current = _.clone(this.attributes);
      }

      // Regular saves `set` attributes before persisting to the server.
      var silentOptions = _.extend({}, options, {silent: true});
      if (attrs && !this.set(attrs, options.wait ? silentOptions : options)) {
        return false;
      }

      // After a successful server-side save, the client is (optionally)
      // updated with the server-side state.
      var model = this;
      var success = options.success;
      options.success = function(resp, status, xhr) {
        var serverAttrs = model.parse(resp, xhr);
        if (options.wait) {
          delete options.wait;
          serverAttrs = _.extend(attrs || {}, serverAttrs);
        }
        if (!model.set(serverAttrs, options)) return false;
        if (success) {
          success(model, resp);
        } else {
          model.trigger('sync', model, resp, options);
        }
      };

      // Finish configuring and sending the Ajax request.
      options.error = Backbone.wrapError(options.error, model, options);
      var method = this.isNew() ? 'create' : 'update';
      var xhr = (this.sync || Backbone.sync).call(this, method, this, options);
      if (options.wait) this.set(current, silentOptions);
      return xhr;
    },

    // Destroy this model on the server if it was already persisted.
    // Optimistically removes the model from its collection, if it has one.
    // If `wait: true` is passed, waits for the server to respond before removal.
    destroy: function(options) {
      options = options ? _.clone(options) : {};
      var model = this;
      var success = options.success;

      var triggerDestroy = function() {
        model.trigger('destroy', model, model.collection, options);
      };

      if (this.isNew()) {
        triggerDestroy();
        return false;
      }

      options.success = function(resp) {
        if (options.wait) triggerDestroy();
        if (success) {
          success(model, resp);
        } else {
          model.trigger('sync', model, resp, options);
        }
      };

      options.error = Backbone.wrapError(options.error, model, options);
      var xhr = (this.sync || Backbone.sync).call(this, 'delete', this, options);
      if (!options.wait) triggerDestroy();
      return xhr;
    },

    // Default URL for the model's representation on the server -- if you're
    // using Backbone's restful methods, override this to change the endpoint
    // that will be called.
    url: function() {
      var base = getValue(this, 'urlRoot') || getValue(this.collection, 'url') || urlError();
      if (this.isNew()) return base;
      return base + (base.charAt(base.length - 1) == '/' ? '' : '/') + encodeURIComponent(this.id);
    },

    // **parse** converts a response into the hash of attributes to be `set` on
    // the model. The default implementation is just to pass the response along.
    parse: function(resp, xhr) {
      return resp;
    },

    // Create a new model with identical attributes to this one.
    clone: function() {
      return new this.constructor(this.attributes);
    },

    // A model is new if it has never been saved to the server, and lacks an id.
    isNew: function() {
      return this.id == null;
    },

    // Call this method to manually fire a `"change"` event for this model and
    // a `"change:attribute"` event for each changed attribute.
    // Calling this will cause all objects observing the model to update.
    change: function(options) {
      options || (options = {});
      var changing = this._changing;
      this._changing = true;

      // Silent changes become pending changes.
      for (var attr in this._silent) this._pending[attr] = true;

      // Silent changes are triggered.
      var changes = _.extend({}, options.changes, this._silent);
      this._silent = {};
      for (var attr in changes) {
        this.trigger('change:' + attr, this, this.get(attr), options);
      }
      if (changing) return this;

      // Continue firing `"change"` events while there are pending changes.
      while (!_.isEmpty(this._pending)) {
        this._pending = {};
        this.trigger('change', this, options);
        // Pending and silent changes still remain.
        for (var attr in this.changed) {
          if (this._pending[attr] || this._silent[attr]) continue;
          delete this.changed[attr];
        }
        this._previousAttributes = _.clone(this.attributes);
      }

      this._changing = false;
      return this;
    },

    // Determine if the model has changed since the last `"change"` event.
    // If you specify an attribute name, determine if that attribute has changed.
    hasChanged: function(attr) {
      if (!arguments.length) return !_.isEmpty(this.changed);
      return _.has(this.changed, attr);
    },

    // Return an object containing all the attributes that have changed, or
    // false if there are no changed attributes. Useful for determining what
    // parts of a view need to be updated and/or what attributes need to be
    // persisted to the server. Unset attributes will be set to undefined.
    // You can also pass an attributes object to diff against the model,
    // determining if there *would be* a change.
    changedAttributes: function(diff) {
      if (!diff) return this.hasChanged() ? _.clone(this.changed) : false;
      var val, changed = false, old = this._previousAttributes;
      for (var attr in diff) {
        if (_.isEqual(old[attr], (val = diff[attr]))) continue;
        (changed || (changed = {}))[attr] = val;
      }
      return changed;
    },

    // Get the previous value of an attribute, recorded at the time the last
    // `"change"` event was fired.
    previous: function(attr) {
      if (!arguments.length || !this._previousAttributes) return null;
      return this._previousAttributes[attr];
    },

    // Get all of the attributes of the model at the time of the previous
    // `"change"` event.
    previousAttributes: function() {
      return _.clone(this._previousAttributes);
    },

    // Check if the model is currently in a valid state. It's only possible to
    // get into an *invalid* state if you're using silent changes.
    isValid: function() {
      return !this.validate(this.attributes);
    },

    // Run validation against the next complete set of model attributes,
    // returning `true` if all is well. If a specific `error` callback has
    // been passed, call that instead of firing the general `"error"` event.
    _validate: function(attrs, options) {
      if (options.silent || !this.validate) return true;
      attrs = _.extend({}, this.attributes, attrs);
      var error = this.validate(attrs, options);
      if (!error) return true;
      if (options && options.error) {
        options.error(this, error, options);
      } else {
        this.trigger('error', this, error, options);
      }
      return false;
    }

  });

  // Backbone.Collection
  // -------------------

  // Provides a standard collection class for our sets of models, ordered
  // or unordered. If a `comparator` is specified, the Collection will maintain
  // its models in sort order, as they're added and removed.
  var Collection = Backbone.Collection = function(models, options) {
    options || (options = {});
    if (options.model) this.model = options.model;
    if (options.comparator) this.comparator = options.comparator;
    this._reset();
    this.initialize.apply(this, arguments);
    if (models) this.reset(models, {silent: true, parse: options.parse});
  };

  // Define the Collection's inheritable methods.
  _.extend(Collection.prototype, Events, {

    // The default model for a collection is just a **Backbone.Model**.
    // This should be overridden in most cases.
    model: Model,

    // Initialize is an empty function by default. Override it with your own
    // initialization logic.
    initialize: function(){},

    // The JSON representation of a Collection is an array of the
    // models' attributes.
    toJSON: function(options) {
      return this.map(function(model){ return model.toJSON(options); });
    },

    // Add a model, or list of models to the set. Pass **silent** to avoid
    // firing the `add` event for every new model.
    add: function(models, options) {
      var i, index, length, model, cid, id, cids = {}, ids = {}, dups = [];
      options || (options = {});
      models = _.isArray(models) ? models.slice() : [models];

      // Begin by turning bare objects into model references, and preventing
      // invalid models or duplicate models from being added.
      for (i = 0, length = models.length; i < length; i++) {
        if (!(model = models[i] = this._prepareModel(models[i], options))) {
          throw new Error("Can't add an invalid model to a collection");
        }
        cid = model.cid;
        id = model.id;
        if (cids[cid] || this._byCid[cid] || ((id != null) && (ids[id] || this._byId[id]))) {
          dups.push(i);
          continue;
        }
        cids[cid] = ids[id] = model;
      }

      // Remove duplicates.
      i = dups.length;
      while (i--) {
        models.splice(dups[i], 1);
      }

      // Listen to added models' events, and index models for lookup by
      // `id` and by `cid`.
      for (i = 0, length = models.length; i < length; i++) {
        (model = models[i]).on('all', this._onModelEvent, this);
        this._byCid[model.cid] = model;
        if (model.id != null) this._byId[model.id] = model;
      }

      // Insert models into the collection, re-sorting if needed, and triggering
      // `add` events unless silenced.
      this.length += length;
      index = options.at != null ? options.at : this.models.length;
      splice.apply(this.models, [index, 0].concat(models));
      if (this.comparator) this.sort({silent: true});
      if (options.silent) return this;
      for (i = 0, length = this.models.length; i < length; i++) {
        if (!cids[(model = this.models[i]).cid]) continue;
        options.index = i;
        model.trigger('add', model, this, options);
      }
      return this;
    },

    // Remove a model, or a list of models from the set. Pass silent to avoid
    // firing the `remove` event for every model removed.
    remove: function(models, options) {
      var i, l, index, model;
      options || (options = {});
      models = _.isArray(models) ? models.slice() : [models];
      for (i = 0, l = models.length; i < l; i++) {
        model = this.getByCid(models[i]) || this.get(models[i]);
        if (!model) continue;
        delete this._byId[model.id];
        delete this._byCid[model.cid];
        index = this.indexOf(model);
        this.models.splice(index, 1);
        this.length--;
        if (!options.silent) {
          options.index = index;
          model.trigger('remove', model, this, options);
        }
        this._removeReference(model);
      }
      return this;
    },

    // Add a model to the end of the collection.
    push: function(model, options) {
      model = this._prepareModel(model, options);
      this.add(model, options);
      return model;
    },

    // Remove a model from the end of the collection.
    pop: function(options) {
      var model = this.at(this.length - 1);
      this.remove(model, options);
      return model;
    },

    // Add a model to the beginning of the collection.
    unshift: function(model, options) {
      model = this._prepareModel(model, options);
      this.add(model, _.extend({at: 0}, options));
      return model;
    },

    // Remove a model from the beginning of the collection.
    shift: function(options) {
      var model = this.at(0);
      this.remove(model, options);
      return model;
    },

    // Get a model from the set by id.
    get: function(id) {
      if (id == null) return void 0;
      return this._byId[id.id != null ? id.id : id];
    },

    // Get a model from the set by client id.
    getByCid: function(cid) {
      return cid && this._byCid[cid.cid || cid];
    },

    // Get the model at the given index.
    at: function(index) {
      return this.models[index];
    },

    // Return models with matching attributes. Useful for simple cases of `filter`.
    where: function(attrs) {
      if (_.isEmpty(attrs)) return [];
      return this.filter(function(model) {
        for (var key in attrs) {
          if (attrs[key] !== model.get(key)) return false;
        }
        return true;
      });
    },

    // Force the collection to re-sort itself. You don't need to call this under
    // normal circumstances, as the set will maintain sort order as each item
    // is added.
    sort: function(options) {
      options || (options = {});
      if (!this.comparator) throw new Error('Cannot sort a set without a comparator');
      var boundComparator = _.bind(this.comparator, this);
      if (this.comparator.length == 1) {
        this.models = this.sortBy(boundComparator);
      } else {
        this.models.sort(boundComparator);
      }
      if (!options.silent) this.trigger('reset', this, options);
      return this;
    },

    // Pluck an attribute from each model in the collection.
    pluck: function(attr) {
      return _.map(this.models, function(model){ return model.get(attr); });
    },

    // When you have more items than you want to add or remove individually,
    // you can reset the entire set with a new list of models, without firing
    // any `add` or `remove` events. Fires `reset` when finished.
    reset: function(models, options) {
      models  || (models = []);
      options || (options = {});
      for (var i = 0, l = this.models.length; i < l; i++) {
        this._removeReference(this.models[i]);
      }
      this._reset();
      this.add(models, _.extend({silent: true}, options));
      if (!options.silent) this.trigger('reset', this, options);
      return this;
    },

    // Fetch the default set of models for this collection, resetting the
    // collection when they arrive. If `add: true` is passed, appends the
    // models to the collection instead of resetting.
    fetch: function(options) {
      options = options ? _.clone(options) : {};
      if (options.parse === undefined) options.parse = true;
      var collection = this;
      var success = options.success;
      options.success = function(resp, status, xhr) {
        collection[options.add ? 'add' : 'reset'](collection.parse(resp, xhr), options);
        if (success) success(collection, resp);
      };
      options.error = Backbone.wrapError(options.error, collection, options);
      return (this.sync || Backbone.sync).call(this, 'read', this, options);
    },

    // Create a new instance of a model in this collection. Add the model to the
    // collection immediately, unless `wait: true` is passed, in which case we
    // wait for the server to agree.
    create: function(model, options) {
      var coll = this;
      options = options ? _.clone(options) : {};
      model = this._prepareModel(model, options);
      if (!model) return false;
      if (!options.wait) coll.add(model, options);
      var success = options.success;
      options.success = function(nextModel, resp, xhr) {
        if (options.wait) coll.add(nextModel, options);
        if (success) {
          success(nextModel, resp);
        } else {
          nextModel.trigger('sync', model, resp, options);
        }
      };
      model.save(null, options);
      return model;
    },

    // **parse** converts a response into a list of models to be added to the
    // collection. The default implementation is just to pass it through.
    parse: function(resp, xhr) {
      return resp;
    },

    // Proxy to _'s chain. Can't be proxied the same way the rest of the
    // underscore methods are proxied because it relies on the underscore
    // constructor.
    chain: function () {
      return _(this.models).chain();
    },

    // Reset all internal state. Called when the collection is reset.
    _reset: function(options) {
      this.length = 0;
      this.models = [];
      this._byId  = {};
      this._byCid = {};
    },

    // Prepare a model or hash of attributes to be added to this collection.
    _prepareModel: function(model, options) {
      options || (options = {});
      if (!(model instanceof Model)) {
        var attrs = model;
        options.collection = this;
        model = new this.model(attrs, options);
        if (!model._validate(model.attributes, options)) model = false;
      } else if (!model.collection) {
        model.collection = this;
      }
      return model;
    },

    // Internal method to remove a model's ties to a collection.
    _removeReference: function(model) {
      if (this == model.collection) {
        delete model.collection;
      }
      model.off('all', this._onModelEvent, this);
    },

    // Internal method called every time a model in the set fires an event.
    // Sets need to update their indexes when models change ids. All other
    // events simply proxy through. "add" and "remove" events that originate
    // in other collections are ignored.
    _onModelEvent: function(event, model, collection, options) {
      if ((event == 'add' || event == 'remove') && collection != this) return;
      if (event == 'destroy') {
        this.remove(model, options);
      }
      if (model && event === 'change:' + model.idAttribute) {
        delete this._byId[model.previous(model.idAttribute)];
        this._byId[model.id] = model;
      }
      this.trigger.apply(this, arguments);
    }

  });

  // Underscore methods that we want to implement on the Collection.
  var methods = ['forEach', 'each', 'map', 'reduce', 'reduceRight', 'find',
    'detect', 'filter', 'select', 'reject', 'every', 'all', 'some', 'any',
    'include', 'contains', 'invoke', 'max', 'min', 'sortBy', 'sortedIndex',
    'toArray', 'size', 'first', 'initial', 'rest', 'last', 'without', 'indexOf',
    'shuffle', 'lastIndexOf', 'isEmpty', 'groupBy'];

  // Mix in each Underscore method as a proxy to `Collection#models`.
  _.each(methods, function(method) {
    Collection.prototype[method] = function() {
      return _[method].apply(_, [this.models].concat(_.toArray(arguments)));
    };
  });

  // Backbone.Router
  // -------------------

  // Routers map faux-URLs to actions, and fire events when routes are
  // matched. Creating a new one sets its `routes` hash, if not set statically.
  var Router = Backbone.Router = function(options) {
    options || (options = {});
    if (options.routes) this.routes = options.routes;
    this._bindRoutes();
    this.initialize.apply(this, arguments);
  };

  // Cached regular expressions for matching named param parts and splatted
  // parts of route strings.
  var namedParam    = /:\w+/g;
  var splatParam    = /\*\w+/g;
  var escapeRegExp  = /[-[\]{}()+?.,\\^$|#\s]/g;

  // Set up all inheritable **Backbone.Router** properties and methods.
  _.extend(Router.prototype, Events, {

    // Initialize is an empty function by default. Override it with your own
    // initialization logic.
    initialize: function(){},

    // Manually bind a single named route to a callback. For example:
    //
    //     this.route('search/:query/p:num', 'search', function(query, num) {
    //       ...
    //     });
    //
    route: function(route, name, callback) {
      Backbone.history || (Backbone.history = new History);
      if (!_.isRegExp(route)) route = this._routeToRegExp(route);
      if (!callback) callback = this[name];
      Backbone.history.route(route, _.bind(function(fragment) {
        var args = this._extractParameters(route, fragment);
        callback && callback.apply(this, args);
        this.trigger.apply(this, ['route:' + name].concat(args));
        Backbone.history.trigger('route', this, name, args);
      }, this));
      return this;
    },

    // Simple proxy to `Backbone.history` to save a fragment into the history.
    navigate: function(fragment, options) {
      Backbone.history.navigate(fragment, options);
    },

    // Bind all defined routes to `Backbone.history`. We have to reverse the
    // order of the routes here to support behavior where the most general
    // routes can be defined at the bottom of the route map.
    _bindRoutes: function() {
      if (!this.routes) return;
      var routes = [];
      for (var route in this.routes) {
        routes.unshift([route, this.routes[route]]);
      }
      for (var i = 0, l = routes.length; i < l; i++) {
        this.route(routes[i][0], routes[i][1], this[routes[i][1]]);
      }
    },

    // Convert a route string into a regular expression, suitable for matching
    // against the current location hash.
    _routeToRegExp: function(route) {
      route = route.replace(escapeRegExp, '\\$&')
                   .replace(namedParam, '([^\/]+)')
                   .replace(splatParam, '(.*?)');
      return new RegExp('^' + route + '$');
    },

    // Given a route, and a URL fragment that it matches, return the array of
    // extracted parameters.
    _extractParameters: function(route, fragment) {
      return route.exec(fragment).slice(1);
    }

  });

  // Backbone.History
  // ----------------

  // Handles cross-browser history management, based on URL fragments. If the
  // browser does not support `onhashchange`, falls back to polling.
  var History = Backbone.History = function() {
    this.handlers = [];
    _.bindAll(this, 'checkUrl');
  };

  // Cached regex for cleaning leading hashes and slashes .
  var routeStripper = /^[#\/]/;

  // Cached regex for detecting MSIE.
  var isExplorer = /msie [\w.]+/;

  // Has the history handling already been started?
  History.started = false;

  // Set up all inheritable **Backbone.History** properties and methods.
  _.extend(History.prototype, Events, {

    // The default interval to poll for hash changes, if necessary, is
    // twenty times a second.
    interval: 50,

    // Gets the true hash value. Cannot use location.hash directly due to bug
    // in Firefox where location.hash will always be decoded.
    getHash: function(windowOverride) {
      var loc = windowOverride ? windowOverride.location : window.location;
      var match = loc.href.match(/#(.*)$/);
      return match ? match[1] : '';
    },

    // Get the cross-browser normalized URL fragment, either from the URL,
    // the hash, or the override.
    getFragment: function(fragment, forcePushState) {
      if (fragment == null) {
        if (this._hasPushState || forcePushState) {
          fragment = window.location.pathname;
          var search = window.location.search;
          if (search) fragment += search;
        } else {
          fragment = this.getHash();
        }
      }
      if (!fragment.indexOf(this.options.root)) fragment = fragment.substr(this.options.root.length);
      return fragment.replace(routeStripper, '');
    },

    // Start the hash change handling, returning `true` if the current URL matches
    // an existing route, and `false` otherwise.
    start: function(options) {
      if (History.started) throw new Error("Backbone.history has already been started");
      History.started = true;

      // Figure out the initial configuration. Do we need an iframe?
      // Is pushState desired ... is it available?
      this.options          = _.extend({}, {root: '/'}, this.options, options);
      this._wantsHashChange = this.options.hashChange !== false;
      this._wantsPushState  = !!this.options.pushState;
      this._hasPushState    = !!(this.options.pushState && window.history && window.history.pushState);
      var fragment          = this.getFragment();
      var docMode           = document.documentMode;
      var oldIE             = (isExplorer.exec(navigator.userAgent.toLowerCase()) && (!docMode || docMode <= 7));

      if (oldIE) {
        this.iframe = $('<iframe src="javascript:0" tabindex="-1" />').hide().appendTo('body')[0].contentWindow;
        this.navigate(fragment);
      }

      // Depending on whether we're using pushState or hashes, and whether
      // 'onhashchange' is supported, determine how we check the URL state.
      if (this._hasPushState) {
        $(window).bind('popstate', this.checkUrl);
      } else if (this._wantsHashChange && ('onhashchange' in window) && !oldIE) {
        $(window).bind('hashchange', this.checkUrl);
      } else if (this._wantsHashChange) {
        this._checkUrlInterval = setInterval(this.checkUrl, this.interval);
      }

      // Determine if we need to change the base url, for a pushState link
      // opened by a non-pushState browser.
      this.fragment = fragment;
      var loc = window.location;
      var atRoot  = loc.pathname == this.options.root;

      // If we've started off with a route from a `pushState`-enabled browser,
      // but we're currently in a browser that doesn't support it...
      if (this._wantsHashChange && this._wantsPushState && !this._hasPushState && !atRoot) {
        this.fragment = this.getFragment(null, true);
        window.location.replace(this.options.root + '#' + this.fragment);
        // Return immediately as browser will do redirect to new url
        return true;

      // Or if we've started out with a hash-based route, but we're currently
      // in a browser where it could be `pushState`-based instead...
      } else if (this._wantsPushState && this._hasPushState && atRoot && loc.hash) {
        this.fragment = this.getHash().replace(routeStripper, '');
        window.history.replaceState({}, document.title, loc.protocol + '//' + loc.host + this.options.root + this.fragment);
      }

      if (!this.options.silent) {
        return this.loadUrl();
      }
    },

    // Disable Backbone.history, perhaps temporarily. Not useful in a real app,
    // but possibly useful for unit testing Routers.
    stop: function() {
      $(window).unbind('popstate', this.checkUrl).unbind('hashchange', this.checkUrl);
      clearInterval(this._checkUrlInterval);
      History.started = false;
    },

    // Add a route to be tested when the fragment changes. Routes added later
    // may override previous routes.
    route: function(route, callback) {
      this.handlers.unshift({route: route, callback: callback});
    },

    // Checks the current URL to see if it has changed, and if it has,
    // calls `loadUrl`, normalizing across the hidden iframe.
    checkUrl: function(e) {
      var current = this.getFragment();
      if (current == this.fragment && this.iframe) current = this.getFragment(this.getHash(this.iframe));
      if (current == this.fragment) return false;
      if (this.iframe) this.navigate(current);
      this.loadUrl() || this.loadUrl(this.getHash());
    },

    // Attempt to load the current URL fragment. If a route succeeds with a
    // match, returns `true`. If no defined routes matches the fragment,
    // returns `false`.
    loadUrl: function(fragmentOverride) {
      var fragment = this.fragment = this.getFragment(fragmentOverride);
      var matched = _.any(this.handlers, function(handler) {
        if (handler.route.test(fragment)) {
          handler.callback(fragment);
          return true;
        }
      });
      return matched;
    },

    // Save a fragment into the hash history, or replace the URL state if the
    // 'replace' option is passed. You are responsible for properly URL-encoding
    // the fragment in advance.
    //
    // The options object can contain `trigger: true` if you wish to have the
    // route callback be fired (not usually desirable), or `replace: true`, if
    // you wish to modify the current URL without adding an entry to the history.
    navigate: function(fragment, options) {
      if (!History.started) return false;
      if (!options || options === true) options = {trigger: options};
      var frag = (fragment || '').replace(routeStripper, '');
      if (this.fragment == frag) return;

      // If pushState is available, we use it to set the fragment as a real URL.
      if (this._hasPushState) {
        if (frag.indexOf(this.options.root) != 0) frag = this.options.root + frag;
        this.fragment = frag;
        window.history[options.replace ? 'replaceState' : 'pushState']({}, document.title, frag);

      // If hash changes haven't been explicitly disabled, update the hash
      // fragment to store history.
      } else if (this._wantsHashChange) {
        this.fragment = frag;
        this._updateHash(window.location, frag, options.replace);
        if (this.iframe && (frag != this.getFragment(this.getHash(this.iframe)))) {
          // Opening and closing the iframe tricks IE7 and earlier to push a history entry on hash-tag change.
          // When replace is true, we don't want this.
          if(!options.replace) this.iframe.document.open().close();
          this._updateHash(this.iframe.location, frag, options.replace);
        }

      // If you've told us that you explicitly don't want fallback hashchange-
      // based history, then `navigate` becomes a page refresh.
      } else {
        window.location.assign(this.options.root + fragment);
      }
      if (options.trigger) this.loadUrl(fragment);
    },

    // Update the hash location, either replacing the current entry, or adding
    // a new one to the browser history.
    _updateHash: function(location, fragment, replace) {
      if (replace) {
        location.replace(location.toString().replace(/(javascript:|#).*$/, '') + '#' + fragment);
      } else {
        location.hash = fragment;
      }
    }
  });

  // Backbone.View
  // -------------

  // Creating a Backbone.View creates its initial element outside of the DOM,
  // if an existing element is not provided...
  var View = Backbone.View = function(options) {
    this.cid = _.uniqueId('view');
    this._configure(options || {});
    this._ensureElement();
    this.initialize.apply(this, arguments);
    this.delegateEvents();
  };

  // Cached regex to split keys for `delegate`.
  var delegateEventSplitter = /^(\S+)\s*(.*)$/;

  // List of view options to be merged as properties.
  var viewOptions = ['model', 'collection', 'el', 'id', 'attributes', 'className', 'tagName'];

  // Set up all inheritable **Backbone.View** properties and methods.
  _.extend(View.prototype, Events, {

    // The default `tagName` of a View's element is `"div"`.
    tagName: 'div',

    // jQuery delegate for element lookup, scoped to DOM elements within the
    // current view. This should be prefered to global lookups where possible.
    $: function(selector) {
      return this.$el.find(selector);
    },

    // Initialize is an empty function by default. Override it with your own
    // initialization logic.
    initialize: function(){},

    // **render** is the core function that your view should override, in order
    // to populate its element (`this.el`), with the appropriate HTML. The
    // convention is for **render** to always return `this`.
    render: function() {
      return this;
    },

    // Remove this view from the DOM. Note that the view isn't present in the
    // DOM by default, so calling this method may be a no-op.
    remove: function() {
      this.$el.remove();
      return this;
    },

    // For small amounts of DOM Elements, where a full-blown template isn't
    // needed, use **make** to manufacture elements, one at a time.
    //
    //     var el = this.make('li', {'class': 'row'}, this.model.escape('title'));
    //
    make: function(tagName, attributes, content) {
      var el = document.createElement(tagName);
      if (attributes) $(el).attr(attributes);
      if (content) $(el).html(content);
      return el;
    },

    // Change the view's element (`this.el` property), including event
    // re-delegation.
    setElement: function(element, delegate) {
      if (this.$el) this.undelegateEvents();
      this.$el = (element instanceof $) ? element : $(element);
      this.el = this.$el[0];
      if (delegate !== false) this.delegateEvents();
      return this;
    },

    // Set callbacks, where `this.events` is a hash of
    //
    // *{"event selector": "callback"}*
    //
    //     {
    //       'mousedown .title':  'edit',
    //       'click .button':     'save'
    //       'click .open':       function(e) { ... }
    //     }
    //
    // pairs. Callbacks will be bound to the view, with `this` set properly.
    // Uses event delegation for efficiency.
    // Omitting the selector binds the event to `this.el`.
    // This only works for delegate-able events: not `focus`, `blur`, and
    // not `change`, `submit`, and `reset` in Internet Explorer.
    delegateEvents: function(events) {
      if (!(events || (events = getValue(this, 'events')))) return;
      this.undelegateEvents();
      for (var key in events) {
        var method = events[key];
        if (!_.isFunction(method)) method = this[events[key]];
        if (!method) throw new Error('Method "' + events[key] + '" does not exist');
        var match = key.match(delegateEventSplitter);
        var eventName = match[1], selector = match[2];
        method = _.bind(method, this);
        eventName += '.delegateEvents' + this.cid;
        if (selector === '') {
          this.$el.bind(eventName, method);
        } else {
          this.$el.delegate(selector, eventName, method);
        }
      }
    },

    // Clears all callbacks previously bound to the view with `delegateEvents`.
    // You usually don't need to use this, but may wish to if you have multiple
    // Backbone views attached to the same DOM element.
    undelegateEvents: function() {
      this.$el.unbind('.delegateEvents' + this.cid);
    },

    // Performs the initial configuration of a View with a set of options.
    // Keys with special meaning *(model, collection, id, className)*, are
    // attached directly to the view.
    _configure: function(options) {
      if (this.options) options = _.extend({}, this.options, options);
      for (var i = 0, l = viewOptions.length; i < l; i++) {
        var attr = viewOptions[i];
        if (options[attr]) this[attr] = options[attr];
      }
      this.options = options;
    },

    // Ensure that the View has a DOM element to render into.
    // If `this.el` is a string, pass it through `$()`, take the first
    // matching element, and re-assign it to `el`. Otherwise, create
    // an element from the `id`, `className` and `tagName` properties.
    _ensureElement: function() {
      if (!this.el) {
        var attrs = getValue(this, 'attributes') || {};
        if (this.id) attrs.id = this.id;
        if (this.className) attrs['class'] = this.className;
        this.setElement(this.make(this.tagName, attrs), false);
      } else {
        this.setElement(this.el, false);
      }
    }

  });

  // The self-propagating extend function that Backbone classes use.
  var extend = function (protoProps, classProps) {
    var child = inherits(this, protoProps, classProps);
    child.extend = this.extend;
    return child;
  };

  // Set up inheritance for the model, collection, and view.
  Model.extend = Collection.extend = Router.extend = View.extend = extend;

  // Backbone.sync
  // -------------

  // Map from CRUD to HTTP for our default `Backbone.sync` implementation.
  var methodMap = {
    'create': 'POST',
    'update': 'PUT',
    'delete': 'DELETE',
    'read':   'GET'
  };

  // Override this function to change the manner in which Backbone persists
  // models to the server. You will be passed the type of request, and the
  // model in question. By default, makes a RESTful Ajax request
  // to the model's `url()`. Some possible customizations could be:
  //
  // * Use `setTimeout` to batch rapid-fire updates into a single request.
  // * Send up the models as XML instead of JSON.
  // * Persist models via WebSockets instead of Ajax.
  //
  // Turn on `Backbone.emulateHTTP` in order to send `PUT` and `DELETE` requests
  // as `POST`, with a `_method` parameter containing the true HTTP method,
  // as well as all requests with the body as `application/x-www-form-urlencoded`
  // instead of `application/json` with the model in a param named `model`.
  // Useful when interfacing with server-side languages like **PHP** that make
  // it difficult to read the body of `PUT` requests.
  Backbone.sync = function(method, model, options) {
    var type = methodMap[method];

    // Default options, unless specified.
    options || (options = {});

    // Default JSON-request options.
    var params = {type: type, dataType: 'json'};

    // Ensure that we have a URL.
    if (!options.url) {
      params.url = getValue(model, 'url') || urlError();
    }

    // Ensure that we have the appropriate request data.
    if (!options.data && model && (method == 'create' || method == 'update')) {
      params.contentType = 'application/json';
      params.data = JSON.stringify(model.toJSON());
    }

    // For older servers, emulate JSON by encoding the request into an HTML-form.
    if (Backbone.emulateJSON) {
      params.contentType = 'application/x-www-form-urlencoded';
      params.data = params.data ? {model: params.data} : {};
    }

    // For older servers, emulate HTTP by mimicking the HTTP method with `_method`
    // And an `X-HTTP-Method-Override` header.
    if (Backbone.emulateHTTP) {
      if (type === 'PUT' || type === 'DELETE') {
        if (Backbone.emulateJSON) params.data._method = type;
        params.type = 'POST';
        params.beforeSend = function(xhr) {
          xhr.setRequestHeader('X-HTTP-Method-Override', type);
        };
      }
    }

    // Don't process data on a non-GET request.
    if (params.type !== 'GET' && !Backbone.emulateJSON) {
      params.processData = false;
    }

    // Make the request, allowing the user to override any Ajax options.
    return $.ajax(_.extend(params, options));
  };

  // Wrap an optional error callback with a fallback error event.
  Backbone.wrapError = function(onError, originalModel, options) {
    return function(model, resp) {
      resp = model === originalModel ? resp : model;
      if (onError) {
        onError(originalModel, resp, options);
      } else {
        originalModel.trigger('error', originalModel, resp, options);
      }
    };
  };

  // Helpers
  // -------

  // Shared empty constructor function to aid in prototype-chain creation.
  var ctor = function(){};

  // Helper function to correctly set up the prototype chain, for subclasses.
  // Similar to `goog.inherits`, but uses a hash of prototype properties and
  // class properties to be extended.
  var inherits = function(parent, protoProps, staticProps) {
    var child;

    // The constructor function for the new subclass is either defined by you
    // (the "constructor" property in your `extend` definition), or defaulted
    // by us to simply call the parent's constructor.
    if (protoProps && protoProps.hasOwnProperty('constructor')) {
      child = protoProps.constructor;
    } else {
      child = function(){ parent.apply(this, arguments); };
    }

    // Inherit class (static) properties from parent.
    _.extend(child, parent);

    // Set the prototype chain to inherit from `parent`, without calling
    // `parent`'s constructor function.
    ctor.prototype = parent.prototype;
    child.prototype = new ctor();

    // Add prototype properties (instance properties) to the subclass,
    // if supplied.
    if (protoProps) _.extend(child.prototype, protoProps);

    // Add static properties to the constructor function, if supplied.
    if (staticProps) _.extend(child, staticProps);

    // Correctly set child's `prototype.constructor`.
    child.prototype.constructor = child;

    // Set a convenience property in case the parent's prototype is needed later.
    child.__super__ = parent.prototype;

    return child;
  };

  // Helper function to get a value from a Backbone object as a property
  // or as a function.
  var getValue = function(object, prop) {
    if (!(object && object[prop])) return null;
    return _.isFunction(object[prop]) ? object[prop]() : object[prop];
  };

  // Throw an error when a URL is needed, and none is supplied.
  var urlError = function() {
    throw new Error('A "url" property or function must be specified');
  };

}).call(this);