def dot(rhs)
      Util3D.check_arg_type(Vector3, rhs)
      self.x*rhs.x + self.y*rhs.y + self.z*rhs.z
    end
def dot(rhs)
      Util3D.check_arg_type(Vector3, rhs)
      self.x*rhs.x + self.y*rhs.y + self.z*rhs.z
    end
[Input]
  _rhs_ should be Vector3.
 [Output]
  return inner product as Numeric