It is fold, although fold is not so great for lists in this context. It's also foldl' union Set.empty, which is better for lists, and probably also for balanced trees. I initially thought that we should surely generalize, but now another alternative comes to mind: remove. As a containers maintainer, I believe we should either:

1. Generalize as proposed, or

2. Deprecate and remove.

I'm currently somewhat in favor of the second option.