Get the set of users that are unique to the given channel (i.e. not
        present in any other channel the bot is in).

def get_unique_users(self, chan):
        """Get the set of users that are unique to the given channel (i.e. not
        present in any other channel the bot is in).
        """
        
        chan = IRCstr(chan)
        
        these_users = self.channels[chan].users
        other_users = set()
        for c in self.channels.values():
            if c.name != chan:
                other_users |= c.users
        
        return these_users - other_users