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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 decr(self, conn, key, decrement=1):
 """Command is used to change data for some item in-place, decrementing it. The data for the item is treated as decimal representation of a 64-bit unsigned integer. :param key: ``bytes``, is the key of the item the client wishes to change :param decrement: ``int``, is the amount by which the client wants to decrease the item. :return: ``int`` new value of the item's data, after the increment or ``None`` to indicate the item with this value was not found """

        assert self._validate_key(key)
        resp = yield from self._incr_decr(
            conn, b'decr', key, decrement)
        return resp