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)
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