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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 clear(self):
 """ Removes all VisItems """

        model = self.tree.model()
        # Don't use model.clear(). it will delete the column sizes
        model.removeRows(0, 1)
        model.setRowCount(1)
        self._setColumnCountForContents()