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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 collapse_all(self):
 """ Collapse all positions; works only if the underlying tree allows it. """

        if implementsCollapseAPI(self._tree):
            self._tree.collapse_all()
            self.set_focus(self._tree.root)
            self._walker.clear_cache()
            self.refresh()