Hot to get parent of current node in walk() #226

I can't realize how to get parent of current node in walk()/match().

My use case is following:

1. Find all inputs.
2. Check it against simple contraints: it must have form in one of it's parents and it must have label among it's siblings.
3. If constraints are not met, throw an error.

Currently I can find inputs with match(), but how do I check it has form parent?