There are a few rather conspicuously missing features: 1. A way to take the intersection of a list of sets. This shouldn't really be a big deal, and it already exists for unions, but the intersection version should probably sort the sets by size before folding, or otherwise try to do something smart. 2. A way to insert an element if an == one is not already present (insert replaces an existing one with a new one). Currently, as far as I can tell, this can only be done using either if e `member` s then s else insert e s which potentially descends the tree twice for no reason or s `union` singleton e which is documented as being O(|s|+1), although I wouldn't be shocked if the documentation were too pessimistic in this case. 3. A way to delete an element and simultaneously find out whether it was in the set. David Feuer