.Set: ClassType
.Set.
    copy: |T|(self: .Set(T)) -> .Set(T)
    differece: |T|(self: .Set(T), other: .Set(T)) -> .Set(T)
    intersection: |T|(self: .Set(T), other: .Set(T)) -> .Set(T)
    isdisjoint: |T|(self: .Set(T), other: .Set(T)) -> Bool
    issubset: |T|(self: .Set(T), other: .Set(T)) -> Bool
    issuperset: |T|(self: .Set(T), other: .Set(T)) -> Bool
    symmetric_difference: |T|(self: .Set(T), other: .Set(T)) -> .Set(T)
    union: |T|(self: .Set(T), other: .Set(T)) -> .Set(T)