wiki有一个对open function的定义，是A function f : X → Y is open if and only if for every x in X and every neighborhood U of x (however small), there exists a neighborhood V of f(x) such that V  f(U)， 没有看明白解释的到底是什么意思，希望有好心的同学帮忙解答一下，非常感谢~