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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 _progress_set_description(self, stage, description):
 """ set description of an already existing progress """

        self.__check_stage_registered(stage)
        self._prog_rep_descriptions[stage] = description
        if self._prog_rep_progressbars[stage]:
            self._prog_rep_progressbars[stage].set_description(description, refresh=False)