How/why does set() in {frozenset()} work?

Question

Even though sets are unhashable, membership check in other set works:

>>> set() in {frozenset()}
True

I expected TypeError:

Answer 1

The last line of the documentation for sets discusses this:

Note, the elem argument to the __contains__(), remove(), and discard() methods may be a set. To support searching for an equivalent frozenset, a temporary one is created from elem.