Not blocking to this PR: IIUC, now any parse operation can return a Nonterminal, right?
I wonder if we should also change the type of ParseOutput::tree() from Node to Nonterminal. Users always pass a NonterminalKind to parse(), so it makes sense to expect a Nonterminal with the same kind back. Thoughts?

Originally posted by @OmarTawfik in #1172 (comment)