docs: def _check_transition_target(self, transition):
          """Checks the validity of a transition target
  
          Checks whether the transition target is valid.
  
          :param rafcon.core.transition.Transition transition: The transition to be checked
          :return bool validity, str message: validity is True, when the transition is valid, False else. message gives
              more information especially if the transition is not valid
          """
  
          to_state_id = transition.to_state
          to_outcome_id = transition.to_outcome
  
          if to_state_id == self.state_id:
              if to_outcome_id not in self.outcomes:
                  return False, "to_outcome is not existing"
          else:
              if to_state_id not in self.states:
                  return False, "to_state is not existing"
              if to_outcome_id is not None:
                  return False, "to_outcome must be None as transition goes to child state"
  
          return True, "valid"