Return the perp product of the given vectors.  The perp product is
        just a cross product where the third dimension is taken to be zero and
        the result is returned as a scalar.

def perp_product(self, other):
        """ Return the perp product of the given vectors.  The perp product is
        just a cross product where the third dimension is taken to be zero and
        the result is returned as a scalar. """

        return self.x * other.y - self.y * other.x