protected function filter()
    {
        $filter = null;

        if (   (!$filter = $this->is("classFilter"))
            && (!$filter = $this->is("idFilter"))
            && (!$filter = $this->is("attrFilter"))
            && (!$filter = $this->is("pseudoFilter"))
        ) {
            return false;
        }
        return $filter;
    }