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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_index(self, start=None):
 """ Seek to the first stage to run. """

        if start is None:
            return 0
        start_stage = translate_stage_name(start)
        internal_names = [translate_stage_name(s.name) for s in self._stages]
        try:
            return internal_names.index(start_stage)
        except ValueError:
            raise UnknownPipelineStageError(start, self)