async def on_isupport_casemapping(self, value):
        """ IRC case mapping for nickname and channel name comparisons. """
        if value in rfc1459.protocol.CASE_MAPPINGS:
            self._case_mapping = value
            self.channels = rfc1459.parsing.NormalizingDict(self.channels, case_mapping=value)
            self.users = rfc1459.parsing.NormalizingDict(self.users, case_mapping=value)
IRC case mapping for nickname and channel name comparisons.