You're giving % higher precedence than *. Are you sure that's correct? According to your grammar, 4*2%3 is equivalent to 4*(2%3) (8) rather than (4*2)%3 (0).

The words op and func are probably redundant. Do you really need to know that something is a op rather than specifically an addition, substratction, multiplication, etc.?

Does extract_quotelike remove the quotes? If so, you have a problem if someone create a string called op or func. That's why I had term in the tree. This is the only level you removed, and it hurts you.

Update: Oops, I'm mistaken on that last one. If it's a ref, it's an op or func. It's it's not, it's a number or string. It's not very orthogonal to flattern terms, but it works for this grammar.