is_emptyniltrueis_emptyconsxlfalsehdconsxlxtlconsxllappendl1l2ifappendl1l2is_emptyl1ifappendl1l2truel2ifappendl1l2falseconshdl1appendtll1l2appendl1l2is_emptyl1appendl1l2ifappendl1l2is_emptyl1ifappendl1l2falsetll1ifappendl1l2falseappendtll1l2ifappendl1l2falsehdl1trueifappendl1l2falseappendtll1l2appendl1l2ifappendl1l2is_emptyl1trueifappendl1l2falsehdl1falseifappendl1l2falsetll1falseappendl1l2is_emptyl1false