Design Add, Overwrite, Undo, and Redo for Billing State

Design an in-memory representation of a billing record that supports three stages of behavior: Add a signed delta to the current balance. Replace the current balance with an explicitly provided value. Undo and redo successful balance changes. The prompt does not say whether history is kept locally, persisted, or shared across sessions, so first state your chosen scope before selecting a representation. Your answer should define the exact behavior of undo and redo, including…