Fix indicator in case of unnanounced departments.

def bump(self):
        """ Fix indicator in case of unnanounced departments. """
        # read client
        values = self.client.mget(self.keys.indicator, self.keys.dispenser)
        indicator, dispenser = map(int, values)

        # determine active users
        numbers = range(indicator, dispenser + 1)
        keys = [self.keys.key(n) for n in numbers]
        pairs = zip(keys, self.client.mget(*keys))

        try:
            # determine number of first active user
            number = next(self.keys.number(key)
                          for key, value in pairs if value is not None)
        except:
            # set number to next result of incr on dispenser
            number = dispenser + 1

        # set indicator to it if necessary
        if number != indicator:
            self.client.set(self.keys.indicator, number)

        # announce and return it anyway
        self.announce(number)
        return number