{"ast_node_type":"function","tokens":[{"end":68,"kind":"keyword","start":0,"text":"fn"}]}
    fn mapper((idx, o): (usize, Option<V>)) -> Option<(Idx<T>, V)> {
impl<T, V> ArenaMapIter<Idx<T>, V> {
        Some((ArenaMap::<Idx<T>, V>::from_idx(idx), o?))