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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 page_prev(self):
 """Go to the previous page."""

        window_start = (self.parent.value('window_start') -
                        self.parent.value('window_length'))
        if window_start < 0:
            return
        self.parent.overview.update_position(window_start)