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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 switch_to_line_in(self, source=None):
 """ Switch the speaker's input to line-in. Args: source (SoCo):
 The speaker whose line-in should be played. Default is line-in from the speaker itself. """

        if source:
            uid = source.uid
        else:
            uid = self.uid

        self.avTransport.SetAVTransportURI([
            ('InstanceID', 0),
            ('CurrentURI', 'x-rincon-stream:{0}'.format(uid)),
            ('CurrentURIMetaData', '')
        ])