def distance(rhs)
      Util3D.check_arg_type(Vector3, rhs)
      Math::sqrt((self.x - rhs.x)*(self.x - rhs.x) + (self.y - rhs.y)*(self.y - rhs.y) + (self.z - rhs.z)*(self.z - rhs.z))
    end
def distance(rhs)
      Util3D.check_arg_type(Vector3, rhs)
      Math::sqrt((self.x - rhs.x)*(self.x - rhs.x) + (self.y - rhs.y)*(self.y - rhs.y) + (self.z - rhs.z)*(self.z - rhs.z))
    end
[Input]
  _rhs_ should be Vector3.
 [Output]
  return distance between two points as Numeric.