Ideal Hash Array Mapped Tries: an Erlang functional datatype

The Hash Array Mapped Trie (HAMT) is based on the simple notion of hashing a
key and storing the key in a trie based on this hash value. The AMT is used to
implement the required structure e#ciently. The Array Mapped Trie (AMT) is a
versatile data structure and yields attractive alternative to contemporary
algorithms in many applications. Here I describe how it is used to develop Hash
Trees with near ideal characteristics that avoid the traditional problem,
setting the size of the initial root hash table or incurring the high cost of
dynamic resizing to achieve an acceptable performance.

Based on the paper "Ideal Hash Tries" by Phil Bagwell [2000].
@ARTICLE{Bagwell01idealhash,
    author = {Phil Bagwell},
    title = {Ideal Hash Trees},
    journal = {Es Grands Champs},
    year = {2001},
    volume = {1195}
}
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.21.6279
http://lampwww.epfl.ch/papers/idealhashtrees.pdf
http://en.wikipedia.org/wiki/Hash_array_mapped_trie

This is an experiment in functional datatypes in Erlang.  At some point I'll
mature this into a persistent concurrent hash array mapped trie (ctrie).