Emacs 24.4 will ship a builtin prettify-symbols-mode:
http://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-24&id=df52f8b
Therefore, I am no longer maintaining this implementation. If you want to use
it anyway, install the pretty-symbols package from melpa and read the
documentation for pretty-symbols-mode, pretty-symbol-patterns, and
pretty-symbol-categories in pretty-symbols.el.