public function add($key, $val) {
        if (!isset($this->_attrs[$key])) {
            $this->_attrs[$key] = [];
        }
        
        if (!strcasecmp($key, 'id')) {
            // value for `id` should be one only
            $this->_attrs[$key] = $val;
        } else {
            // else append it
            array_push($this->_attrs[$key], $val);
        }
        
        return $this;
    }