function resolve( $url = '', $query_string_vars = [ ] ) {
        $url = filter_var( $url, FILTER_SANITIZE_URL );
        if ( ! empty( $url ) ) {
            $id = md5( $url . serialize( $query_string_vars ) );
            if ( ! isset( $this->items[$id] ) || ! $this->items[$id] instanceof UrlToQueryItem ) {
                $this->items[$id] = new UrlToQueryItem( $this );
            }
            $result = $this->items[$id]->resolve( $url, $query_string_vars );
        } else {
            $result = new \WP_Error( 'url-to-query-bad-url' );
        }
        return $result;
    }