Please just use an `Option<usize>`, not that wonky NULL thing. If you're worried about the slightly more memory that uses, you can use `Option<NonZeroUsize>` a

Please just use an `Option<usize>`, not that wonky NULL thing. If you're worried about the slightly more memory that uses, you can use `Option<NonZeroUsize>` and subtract 1 for all indexing operations.