Point = Class {x = Int; y = Int}
Point.
    new x, y = Point {x; y}
    norm self = self::x**2 + self::y**2
Point|<: Add(Point)|.
    Output = Point
    # This is same as `__add__ self, other: Point = ...`
    `_+_` self, other: Point =
        Point.new(self::x + other::x, self::y + other::y)
Point|<: Mul(Point)|.
    Output = Int
    `*` self, other: Point =
        self::x * other::x + self::y * other::y
Point|<: 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: Int = p * q
assert s == 11
assert r == Point.new 4, 6
assert r.norm() == 52

MyList = Class {
    .list = List(Obj)
}
MyList|<: Iterable(Obj)|.
    Iter = ListIterator(Obj)
    iter self = self.list.iter()
MyList|<: Sized|.
    __len__ self = len self.list
MyList|<: Container(Obj)|.
    __contains__ self, x: Obj = x in self.list
MyList|<: Sequence(Obj)|.
    __getitem__ self, idx = self.list[idx]

MyList2 = Class {
    .list = List(Obj)
}
MyList2|<: Sequence(Obj)|.
    Iter = ListIterator(Obj)
    iter self = self.list.iter()
    __contains__ self, x: Obj = x in self.list
    __len__ self = len self.list
    __getitem__ self, idx = self.list[idx]