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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 _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)