code: @Override
  	public synchronized ID removeOnline(final USER user)
  	{
  		final ID sessionId = super.removeOnline(user);
  		if (sessionId != null)
  		{
  			sessionIdToSession.remove(sessionId);
  		}
  		return sessionId;
  	}
docs: Removes the user from the map. This method shell be invoked when the session is unbounded
  from the Application. In wicket is the best way to do that in the
  {@code WebApplication#sessionUnbound(String)}.
  
  @param user
  the user
  @return the session id