def positions(self, reverse=False):
        """returns a generator that walks the positions of this tree in DFO"""
        def Posgen(reverse):
            if reverse:
                lastrootsib = self.last_sibling_position(self.root)
                current = self.last_decendant(lastrootsib)
                while current is not None:
                    yield current
                    current = self.prev_position(current)
            else:
                current = self.root
                while current is not None:
                    yield current
                    current = self.next_position(current)
        return Posgen(reverse)
returns a generator that walks the positions of this tree in DFO