code: def _start_index(self, start=None):
          
          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)
docs: Seek to the first stage to run.