Not just Scheme, but given how important metaprogramming is to Python, I was hoping there would be more Lisp dialects in the list.
The ABC module, collections, frozenset. Basically anywhere we add custom types or generate new classes to instantiate.
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!