Appears to be a 16 way branching trie which completely misses both advantages of tree structures over hashes: 1/ This tree is mutable, insert doesn't give you

Appears to be a 16 way branching trie which completely misses both advantages of tree structures over hashes: 1/ This tree is mutable, insert doesn't give you a new tree via path copying 2/ union/intersection style operations can be sublinear. None of the batch operations are implemented