public User registerUser(String email, String password, String fullName) throws EmailAlreadyBoundException {
        User existing = null;
        try {
            existing = users.byEmail(email);
            throw new EmailAlreadyBoundException(email);
        } catch (NoEmailException e) {
            User user = new User(email, password, fullName);
            users.put(user);
            return user;
        }
    }