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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 _start(self, start_message):
 """ Start this action given its start message. @param WrittenMessage start_message: A start message that has the same level as this action. @raise InvalidStartMessage: If C{start_message} does not have a C{ACTION_STATUS_FIELD} of C{STARTED_STATUS}, or if it has a C{task_level} indicating that it is not the first message of an action. """

        if start_message.contents.get(
            ACTION_STATUS_FIELD, None) != STARTED_STATUS:
            raise InvalidStartMessage.wrong_status(start_message)
        if start_message.task_level.level[-1] != 1:
            raise InvalidStartMessage.wrong_task_level(start_message)
        return self.set(start_message=start_message)