<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def run_zone(self, minutes, zone=None):
 """ Run or stop a zone or all zones for an amount of time. :param minutes: The number of minutes to run. :type minutes: int :param zone: The zone number to run. If no zone is specified then run all zones. :type zone: int or None :returns: The response from set_zones() or None if there was an error. :rtype: None or string """

        if zone is None:
            zone_cmd = 'runall'
            relay_id = None
        else:
            if zone < 0 or zone > (len(self.relays) - 1):
                return None
            else:
                zone_cmd = 'run'
                relay_id = self.relays[zone]['relay_id']

        if minutes <= 0:
            time_cmd = 0
            if zone is None:
                zone_cmd = 'stopall'
            else:
                zone_cmd = 'stop'
        else:
            time_cmd = minutes * 60

        return set_zones(self._user_token, zone_cmd, relay_id, time_cmd)