Point = Class {x = Int; y = Int}
Point.
    new x, y = Point::__new__ {x; y}
    norm self = self::x**2 + self::y**2
Point|Point <: Add(Point)|.
    Output = Point
    __add__ self, other: Point =
        Point.new(self::x + other::x, self::y + other::y)
Point|Point <: Mul(Point)|.
    Output = Nat
    __mul__ self, other: Point =
        self::x * other::x + self::y * other::y
Point|Point <: Eq|.
    __eq__ self, other: Point =
        self::x == other::x and self::y == other::y

p = Point.new 1, 2

q = Point.new 3, 4

r: Point = p + q
s: Nat = p * q
assert s == 11
assert r == Point.new 4, 6
assert r.norm() == 52