[0]:以前初步的了解了yield，今天在稍微深入的学习一下.

[1]首先含有yield关键字的函数就是协程,也是generator，注意,这个generator不再是函数的调用方式了,具体看一个例子

class Node(object):
    def __init__(self,value):
        self._left = []
        self._right = []
        self.value = value
    def iterate(self):
        for node in self._left:
            for value in node.iterate():yield value
        yield self.value
        for node in self._right:
            for value in node.iterate():yield value
    def dfs(self):