Usernames Directory

You need to implement a directory for usernames where users can register with unique usernames, and the system should support checking for available usernames. Create a class `UsernameDirectory` with the following methods: 1. `register(username: str)`: Registers a new username if it's available and returns 'Successfully registered' or 'Username already taken'. 2. `is_available(username: str) -> bool`: Checks if a username is available. The commands come in the format: 'REGISTER username' or 'CHECK username'. The input ends with 'END'. Input Format: - Each command is on a new line, and the last line contains 'END'. Output Format: - For each 'REGISTER' command, print the registration status. For each 'CHECK' command, print 'Available' or 'Taken'. Example: Input: REGISTER alice REGISTER bob CHECK alice CHECK charlie REGISTER charlie END Output: Successfully registered Successfully registered Taken Available Successfully registered