Design an object-oriented shopping cart module for an e-commerce system. Requirements: - A cart can **add items** and **update item quantities**. - The cart supports **checkout**. After checkout, the cart becomes **immutable** (no further changes to items/quantities/coupons). - The cart can **apply coupons**. There may be multiple coupon types, for example: - **Percentage off** (e.g., 20% off) - **BOGO** (Buy-One-Get-One) style discounts - The cart must compute **tax** based on a **state tax rate**. - The prompt is intentionally vague: explain what clarifying questions you would ask, then propose a clean class/interface design and describe how totals are calculated. Deliverables: - Key classes and responsibilities (e.g., Cart, LineItem, Product, Pricing/Discount/Tax components). - Core methods and data structures. - How you prevent modification after checkout. - How you model different coupon types and make it easy to add new ones.