Very cool! I think you've inspired me to implement something like `Expression << Expression` much like your `$`.
I'm currently trying to determine the best way to implement `From<Expression> for u64` (or maybe `Option<u64>`...), so I can convert both to and from `u64` types as church encoded numerals. Eventually the goal is to `impl Into/From<Expression>` for all the types one might use, giving a horribly inefficient runtime for Rust ;)
