@Override
 public void removeUsers(User user) {
   if (user.isRoot() || user.isGuest()) {
     throw new IllegalRemovalException("You can't remove " + user.getName());
   } else {
     super.removeUsers(user);
   }
 }
 protected void removeUsers(String username) throws IllegalRemovalException, UserUnknownException {
   hasUser(username);
   if (username.equals(ROOT_USER)) throw new IllegalRemovalException(username);
   else {
     User toRemove = getUserByUsername(username);
     toRemove.remove();
     super.removeUsers(toRemove);
   }
 }