But at least they shouldn't use phrases like `persistent sets` in the documentation. I confused that with the datatype on my first reading. Technical details ar

But at least they shouldn't use phrases like `persistent sets` in the documentation. I confused that with the datatype on my first reading. Technical details are necessary but that is just creating unnecessary confusion. Compare that to the haskell documentation: A map from hashable keys to values. A map cannot contain duplicate keys; each key can map to at most one value. A HashMap makes no guarantees as to the order of its elements. The implementation is based on hash array mapped tries. A HashMap is often faster than other tree-based set types, especially when key comparison is expensive, as in the case of strings. Many operations have a average-case complexity of O(log n). The implementation uses a large base (i.e. 16) so in practice these operations are constant time.