Design an Automated Teller Machine (ATM) system. The system should allow users to perform basic banking operations such as depositing cash, withdrawing cash, checking balance, and transferring funds. The system should also be designed with security and concurrency in mind. Imagine you're designing this for a bank that wants a robust, scalable, and maintainable solution. Focus on low-level design principles, object-oriented design, and design patterns to create a flexible and extensible system.

Requirements

Main Classes/Interfaces

*   **ATM:** Represents the ATM machine itself. This is the main entry point for user interactions.  It should manage the overall workflow, including authentication, transaction processing, and dispensing cash.  It needs to be thread-safe.
*   **CardReader:**  Handles reading the user's card. Abstract this to allow for different card reader implementations.
*   **Authenticator:** Handles user authentication (PIN verification, account locking).  Consider different authentication mechanisms in the future (biometrics).
*   **Account:** Represents a user's bank account.  Holds account balance and transaction history. Account information should be protected from concurrent access.
*   **Transaction:** An abstract class or interface for different transaction types (Deposit, Withdrawal, Transfer).  Use a pattern to create and process transactions.
*   **TransactionProcessor:**  Processes different types of transactions.  Consider how to extend this to support new transaction types.
*   **CashDispenser:**  Manages the ATM's cash inventory and dispenses cash in the correct denominations. Design this for fault tolerance if a particular denomination is empty.
*   **DepositSlot:** Handles cash deposits, validating the bills.
*   **Currency:** Represents a currency and its denominations.
*   **AuditLogger:** Logs all transactions for auditing purposes. Use a suitable pattern to ensure transactions are reliably logged.