Implement a Transactional Key-Value Store

Design an in-memory key-value store that supports transactions.

Required operations:

• set(key, value): assign or overwrite the value for a key.
• get(key): return the current value for the key, or null if it does not exist.
• count(value): return how many keys are currently mapped to this value.
• begin(): start a new transaction.
• commit(): persist the changes from all currently open transactions.
• rollback(): undo the changes from the most recent open transaction.

Assumptions:

• Keys are unique strings.
• Values can be any hashable scalar.
• Multiple transactions may be nested.
• count(value) must reflect the currently visible state, including uncommitted changes inside the active transaction.
• If rollback() or commit() is called with no open transaction, define and handle the behavior clearly.

Discuss the data structures you would use and implement the core API.