def __mod__(self, v):
        '''cross product'''
        return Vector3(self.y*v.z - self.z*v.y,
                       self.z*v.x - self.x*v.z,
                       self.x*v.y - self.y*v.x)
def __mod__ ( self , v ) : return Vector3 ( self . y * v . z - self . z * v . y , self . z * v . x - self . x * v . z , self . x * v . y - self . y * v . x )
cross product