public function popKey()
    {
        if (strlen($this->base)) {
            $layers = explode($this->delimiter, $this->base);
            $last = count($layers) - 1;
            $layer = $layers[$last];
            if ($last > 0) {
                unset ($layers[$last]);
                $this->base = implode($this->delimiter, $layers);
            } else {
                $this->base = '';
            }
            return $layer;
        }

        return null;
    }