There are a few classes in the FSet codebase that I don’t consider ready for general use, even though they’ve been there for years. Either they aren’t well tested, or I don’t feel they’re designed quite right, or they don’t have all the operations I think people would want, or some combination of these. You should not consider them part of the stable API. But, as I say, they’ve been lying around for a long time, and a few people may have used them, so I feel I should mention them briefly. See the code for the details.