IO = Trait {
    .Inp = Type;
    .Output = Type;
    .func = (self, x: .Inp) -> .Output
}

C = Class()
C|<: IO|.
    Inp = Int
    Output = Bool
    func self, i =
        _ = self
        i >= 0

io x, y = x.func y

assert io C.new(), 1