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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 set_stage_index(self):
 """Set the current stage in combobox."""

        window_start = self.parent.value('window_start')
        window_length = self.parent.value('window_length')
        stage = self.annot.get_stage_for_epoch(window_start, window_length)
        #lg.info('winstart: ' + str(window_start) + ', stage: ' + str(stage))

        if stage is None:
            self.idx_stage.setCurrentIndex(-1)
        else:
            self.idx_stage.setCurrentIndex(STAGE_NAME.index(stage))