Lookahead and lookback

[Veykril]

I haven't looked into this too much yet, but if I am not mistaken the quote crate uses a neat trick to allow lookahead and lookback-ish matching on tokens, see:
https://github.com/dtolnay/quote/blob/278057d0d2e59c7de95d79d42e4934783d013e83/src/lib.rs#L672-L702

it might be nice to add this to the pattern section(or do a case study on the macro even?)

[Veykril]

There is no a comment explaining this: https://github.com/dtolnay/quote/blob/31c3be473d0457e29c4f47ab9cff73498ac804a7/src/lib.rs#L664-L746