docs: def _check_start_transition(self, start_transition):
          """Checks the validity of a start transition
  
          Checks whether the given transition is a start transition a whether it is the only one within the state.
  
          :param rafcon.core.transition.Transition start_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
          """
          for transition in self.transitions.values():
              if transition.from_state is None:
                  if start_transition is not transition:
                      return False, "Only one start transition is allowed"
  
          if start_transition.from_outcome is not None:
              return False, "from_outcome must not be set in start transition"
  
          return self._check_transition_target(start_transition)