def clear(self):
        ''' Remove all content from the document but do not reset title.

        Returns:
            None

        '''
        self._push_all_models_freeze()
        try:
            while len(self._roots) > 0:
                r = next(iter(self._roots))
                self.remove_root(r)
        finally:
            self._pop_all_models_freeze()
Remove all content from the document but do not reset title.

        Returns:
            None