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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 go_previous_thumbnail(self):
 """Select the thumbnail previous to the currently selected one."""

        if self.current_thumbnail is not None:
            index = self._thumbnails.index(self.current_thumbnail) - 1
            index = index if index >= 0 else len(self._thumbnails) - 1
            self.set_current_index(index)
            self.scroll_to_item(index)