protected function _sub_ids($eq = false)
    {
        $ret = array();
        $ce  = reset($this->ids);
        $cb  = key($this->ids);
        $doc = $this->doc();
        foreach ($doc->ids as $b => $e) {
            if ($b < $cb || !$eq && $b == $cb) {
                continue;
            }

            if ($b < $ce) {
                $ret[$b] = $e;
            } else {
                $ce = next($this->ids);
                if (!$ce) {
                    break;
                }
                // end of context
                $cb = key($this->ids);
            }
        }
        return $ret;
    }