def cross(rhs)
      Util3D.check_arg_type(Vector3, rhs)
      Vector3.new(
              self.y*rhs.z - self.z*rhs.y,
              self.z*rhs.x - self.x*rhs.z,
              self.x*rhs.y - self.y*rhs.x)
    end
def cross(rhs)
      Util3D.check_arg_type(Vector3, rhs)
      Vector3.new(
              self.y*rhs.z - self.z*rhs.y,
              self.z*rhs.x - self.x*rhs.z,
              self.x*rhs.y - self.y*rhs.x)
    end
[Input]
  _rhs_ should be Vector3.
 [Output]
  return cross product as Vector3.