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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_dag(self, dag, *, data=None):
 """ Schedule the execution of a dag by sending a signal to the workflow. Args: dag (Dag, str):
 The dag object or the name of the dag that should be started. data (MultiTaskData):
 The data that should be passed on to the new dag. Returns: str: The name of the successfully started dag. """

        return self._client.send(
            Request(
                action='start_dag',
                payload={'name': dag.name if isinstance(dag, Dag) else dag,
                         'data': data if isinstance(data, MultiTaskData) else None}
            )
        ).payload['dag_name']