code: def insert(self, song):
          
          if song in self._songs:
              return
          if self._current_song is None:
              self._songs.append(song)
          else:
              index = self._songs.index(self._current_song)
              self._songs.insert(index + 1, song)
docs: 在当前歌曲后插入一首歌曲