docs: def _check_transition_id(self, transition):
          """Checks the validity of a transition id
  
          Checks whether the transition id is already used by another transition within the state
  
          :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
          """
          transition_id = transition.transition_id
          if transition_id in self.transitions and transition is not self.transitions[transition_id]:
              return False, "transition_id already existing"
          return True, "valid"