class BankAccount {
  int balance_;
public:
     void Deposit(int amount, StrictLock<BankAccount>&) {
    // Externally locked
    balance_ += amount;
  }
  void Deposit(int amount) {
    Lock guard(*this); // Internally locked
    Deposit(amount, guard);
  }
    void Withdraw(int amount, StrictLock<BankAccount>&) {
    // Externally locked
    balance_ -= amount;
  }
  void Withdraw(int amount) {
    Lock guard(*this); // Internally locked
    Withdraw(amount, guard);
  }
};