> unsorted_set::erase should be O(1) complexity in time. It also returns
> a pointer to the next element in the data structure. For most
> implementations of a hashtable, this means iterating through all the
> empty buckets looking for the next element. If the hashtable is empty,
> this means checking every single bucket. If there's a large number of
> buckets, this takes a really long time.

> Of course this is implementation specific, but most implementations I
> can think of would be affected by this.