- add more tests
- support XPath functions with CSS selectors
support optionality operator ("?")- support complete arrays with the "!" operator (https://github.com/fizx/parsley/wiki/JSON-Structure#requiring-complete-arrays-with-the--operator)
- support bucketed arrays (https://github.com/fizx/parsley/wiki/JSON-Structure#bucketed-arrays); see https://github.com/redapple/parslepy/wiki/Implementing-bucketed-arrays-(work-in-progess)
- investigate PyParsley API