That's an oops on my part. Chalk it up to a baby who likes to wake up at 4:45am and a toddler who wakes up at 7. frozenset is just a built-in immutable set typ

That's an oops on my part. Chalk it up to a baby who likes to wake up at 4:45am and a toddler who wakes up at 7. frozenset is just a built-in immutable set type with a nice hashing algorithm for the kinds of workloads you'd expect. It's defined in Objects/setobject.c and is a nice read. I was just rambling and I've used frozenset in metaprogramming code when constructing classes via decorators to implement Lattice types with decorated monotone/morphism methods. Sorry for the confusion!