check if nodes inside expressions exist?

[MartijnR]

limitation would be external data of course, but for non-external data it would be helpful

[MartijnR]

This could also serve as an alternative solution to #106

[MartijnR]

It might be a bit of a challenge to write a lexer. We have one in openrosa-xpath-evaluator though. This could perhaps be a functionality that is built-in to the evaluator. Instead of evaluate(), we could add the ability to find_internal_missing_nodes() or something that returns an array of missing intern nodes that are referred to in the expression.