.C = Class()
.C|.C <: Eq|.
    __eq__ self, other: .C =
        _ = self
        _ = other
        True
.C|.C <: Add(Nat)|.
    Output = Nat
    __add__ self, other: Nat =
        _ = self
        other
.C|.C <: Add(Int)|.
    Output = .C
    __add__ self, other: Int =
        _ = other
        self

c = .C.new()
assert c + 1 == 1
assert c + -1 == c