private function getSourceExcerpt($source='', $line=1, $plus=1, $format=false)
    {
        $excerpt = [];
        $twig  = Escape::html($source);
        $lines = preg_split("/(\r\n|\n|\r)/", $twig);
        $start = max(1, $line - $plus);
        $limit = min(count($lines), $line + $plus);
        for ($i = $start - 1; $i < $limit; $i++) {
            if ($format) {
                $attr = 'data-line="'.($i+1).'"';
                if ($i === $line - 1) $excerpt[] = "<mark $attr>$lines[$i]</mark>";
                else $excerpt[] = "<span $attr>$lines[$i]</span>";
            }
            else {
                $excerpt[] = $lines[$i];
            }
        }
        return implode("\n", $excerpt);
    }