code: public synchronized USER remove(final ID sessionId)
  	{
  		final USER user = getUser(sessionId);
  		if (user != null)
  		{
  			usersOnline.remove(user);
  		}
  		sessionIdToUser.remove(sessionId);
  		return user;
  	}
docs: Removes the user from the map with the session id.
  
  @param sessionId
  the session id
  @return the user