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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def on_song_changed(self, song):
 """bind song changed signal with this"""

        if song is None or song.lyric is None:
            self._lyric = None
            self._pos_s_map = {}
        else:
            self._lyric = song.lyric.content
            self._pos_s_map = parse(self._lyric)
        self._pos_list = sorted(list(self._pos_s_map.keys()))
        self._pos = None
        self.current_sentence = ''