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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 on_all_ok(self):
 """ This method is called when all tasks reach S_OK Ir runs `mrgddb` in sequential on the local machine to produce the final DDB file in the outdir of the `Work`. """

        # Merge DDB files.
        out_ddb = self.merge_ddb_files()
        return self.Results(node=self, returncode=0, message="DDB merge done")