def _createtoken(self, type_, value, flags=None):
        '''create a token with position information'''
        pos = None
        assert len(self._positions) >= 2, (type_, value)
        p2 = self._positions.pop()
        p1 = self._positions.pop()
        pos = [p1, p2]
        return token(type_, value, pos, flags)
create a token with position information