__add__ : Implements the plus "+" operator. __sub__ : Implements the minus "-" operator. __mul__ : Implements the multiplication "*" operator. __div__ : Implements the division "/" operator.
class Double(object) : def __init__(self) : self.value = 0
def __add__(self, value) : return self.value + 2 * value
def __sub__(self, value) : return self.value - 2 * value