```python def register_user(username, password): """ Registers a new user. Args: username (str): The username of the new user. password (str): The password of the new user. Raises: ValueError: If the username is already taken. """ Check if the username is already taken if username in users: raise ValueError("Username already taken") Create a new user object user = User(username, password) Add the new user to the list of users users.append(user) ``` J影音a: ```j影音a public void registerUser(String username, String password) { // Check if the username is already taken if (users.containsKey(username)) { throw new IllegalArgumentException("Username already taken"); } // Create a new user object User user = new User(username, password); // Add the new user to the map of users users.put(username, user); } ``` C: ```csharp public void RegisterUser(string username, string password) { // Check if the username is already taken if (users.ContainsKey(username)) { throw new ArgumentException("Username already taken"); } // Create a new user object User user = new User(username, password); // Add the new user to the dictionary of users users.Add(username, user); } ``` PHP: ```php public function registerUser(string $username, string $password) { // Check if the username is already taken if (isset($this->users[$username])) { throw new Exception("Username already taken"); } // Create a new user object $user = new User($username, $password); // Add the new user to the array of users $this->users[$username] = $user; } ```