The hash-map trie is convenient to work with at the REPL since the
key/values are human readable and the backing data is traversible in a
fashion familiar to Clojure.
The byte-array backed trie has a slightly different API but is far more
memory efficient.
The paper that the tightly backed trie is based on can be viewed at
https://www.aclweb.org/anthology/W09-1505.pdf