{"ast_node_type":"function","tokens":[{"end":52,"kind":"keyword","start":0,"text":"fn"}]}
    fn index_mut(&mut self, idx: Idx<V>) -> &mut T {
impl<T, V> std::ops::IndexMut<Idx<V>> for ArenaMap<Idx<V>, T> {
        self.v[Self::to_idx(idx)].as_mut().unwrap()