This is a (partial) implementation of a Map interface on integers, except that it internally uses persistent arrays. This ensures O(1) accesses in non-backtracking cases. It is thus better suited for zero-starting, contiguous keys, or otherwise a lot of space will be empty. To keep track of the present keys, a binary tree is also used, so that adding a key is still logarithmic. It is therefore essential that most of the operations are accesses and not add/removes.