$_ binding

[kazcw]

$_ is a special binding that:

1. doesn't require a type specification
2. doesn't constrain multiple $_ in a pattern to be equal

Because metavars need to be transformed to placeholder expressions of the appropriate type before parsing in a non extend-parser implementation, (1) is a little tricky. I'm thinking of just trying a parse of each possible metavar type and going with the most general that succeeds (i.e. capture the highest possible node in the ast).