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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:
 def start_trajectory(self, trajectory_id, time_scale=1.0, relative=False, reversed=False, group_mask=ALL_GROUPS):
 """ starts executing a specified trajectory :param trajectory_id: id of the trajectory (previously defined by define_trajectory) :param time_scale: time factor; 1.0 = original speed; >1.0: slower; <1.0: faster :param relative: set to True, if trajectory should be shifted to current setpoint :param reversed: set to True, if trajectory should be executed in reverse :param group_mask: mask for which CFs this should apply to :return: """

        self._send_packet(struct.pack('<BBBBBf',
                                      self.COMMAND_START_TRAJECTORY,
                                      group_mask,
                                      relative,
                                      reversed,
                                      trajectory_id,
                                      time_scale))