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.