public function push($varName, $tail)
    {
        if (false === $this->has($varName)) {
            $this->set($varName, null);
        }
        if (!isset($this->varStack[json_encode($varName)])) {
            $this->varStack[json_encode($varName)] = array();
        }
        array_push($this->varStack[json_encode($varName)], $this->get($varName));
        $this->set($varName, $tail);
    }