protected function extractInterpolation($list) {
        $items = $list[2];
        foreach ($items as $i => $item) {
            if ($item[0] == "interpolate") {
                $before = array("list", $list[1], array_slice($items, 0, $i));
                $after = array("list", $list[1], array_slice($items, $i + 1));
                return array("interpolated", $item, $before, $after);
            }
        }
        return $list;
    }