The attached patch implements the Contains algorithm for RecursivePrefixTreeStrategy; includes tests.
Unlike Intersects & Within, this one doesn't share the same tree visiting code, but it is also a recursive trie algorithm. I expect for typical use cases that it'll be quite scalable and fast, though it does have to traverse all the way to the bottom detail at least once.
The existing test infrastructure made testing this easy. The ~260 lines of often complicated code that implements the algorithm and miraculously it passed right away. I upped the test repetitions to 1000 and set some break points at key points and indeed, it appears to work. Today is a good day
Ryan McKinley, you seemed interested in getting ahold of this algorithm. Do you want to test it on your data?
I'll commit this in a couple days.