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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:
 async def on_isupport_excepts(self, value):
 """ Server allows ban exceptions. """

        if not value:
            value = BAN_EXCEPT_MODE
        self._channel_modes.add(value)
        self._channel_modes_behaviour[rfc1459.protocol.BEHAVIOUR_LIST].add(value)