Js_of_ocaml_compiler.Int_setPersistent sets of non-negative integers.
The set is a Patricia tree (a binary trie indexed by the bits of the elements, with path compression) whose leaves are small bitmaps. A set of a few elements is just a few small blocks, whatever the elements, and dense sets are packed 32 elements per leaf. Set operations (union, inter, diff, equal) work structurally. The interface is a subset of Set.S (extended as in Stdlib.Set.S), with the same semantics; in particular, the elements are always visited in increasing order. The only difference is that the elements must be non-negative: add raises Invalid_argument on a negative one.