That&#x27;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.<p><i>frozenset</i> is just a built-in immutable set type with a nice hashing algorithm for the kinds of workloads you&#x27;d expect. It&#x27;s defined in Objects&#x2F;setobject.c and is a nice read.<p>I was just rambling and I&#x27;ve used frozenset in metaprogramming code when constructing classes via decorators to implement Lattice types with decorated monotone&#x2F;morphism methods.<p>Sorry for the confusion!