Keep the Index Honest

Extend the repository idea: an indexed store where every write maintains a secondary index. Implement in Solution: - record Book(String isbn, String genre) - static class IndexedStore with save(Book b) (upsert), List<Book> byGenre(String genre) (insertion order), int size(), and remove(String isbn) (boolean). The index is the contract: byGenre must NEVER return a book that was updated to another genre, re-saved, or removed. Upserting must move the book between index buckets — the exact bug from the lesson.

Difficulty: intermediate