Initializer for empty sets.
Sets have a count.
Set union doesn’t require `inout`.
Return the element directly.
Implement reduce() on sets.
Try not to expose implementation details in types.
Add a Unit type.
Sets are [Element : Unit].
Unit is Equatable.
Sets of Equatable elements can be equated.

They are not Equatable because extensional/intensional.
Tidy up the tests.
Test unions.