assertEquals("user3", chain.changePasswordCalls.get(0).getKey());
        assertEquals(user1, chain.loadCalls.get(0));
    }

    // Test concurrent operations simulation
    public void test_implementation_sequentialOperations() {
        TestAuthenticationChain chain = new TestAuthenticationChain();
        List<User> users = new ArrayList<>();

        // Create test users
        for (int i = 0; i < 10; i++) {