def level(self, level):
        if level == 0 or self.value() == None:
            return [self.value()]
        else:
            return self.lc().level(level-1) + self.rc().level(level-1)



    def bfs_order_star(self):
        '''
        Returns a list of all members in breadth-first search* order, which
        means that empty nodes are denoted by "stars" (here the value None).

        For example, consider the following tree `t`:
                    10
              5           15
           *     *     *     20

        The output of t.bfs_order_star() should be:
        [ 10, 5, 15, None, None, None, 20 ]
        '''
        h=self.height()
        arr=[]
        for i in range(h):
            arr=arr+self.level(i)
        return arr