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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 _removeBackrefs( senderkey):
 """Remove all back-references to this senderkey"""

    try:
        signals = connections[senderkey]
    except KeyError:
        signals = None
    else:
        items = signals.items()
        def allReceivers( ):
            for signal,set in items:
                for item in set:
                    yield item
        for receiver in allReceivers():
            _killBackref( receiver, senderkey )