next

        pub fn next(it: *FieldIterator) ?Field {
            const idx = it.next_idx;
            if (idx == it.names.len) return null;
            it.next_idx += 1;
            return .{
                .idx = idx,
                .name = it.names[idx],
                .value_body = it.body(if (it.value_body_lens) |l| l[idx] else 0),
            };
        }