public void addKnownId(URI id){
  if (_lastKnownUserIds == null) {
    _lastKnownUserIds=new StringSet();
  }
  _lastKnownUserIds.add(id.toString());
}

public void addKnownId ( URI id ) { if ( _lastKnownUserIds == null ) { _lastKnownUserIds = new StringSet ( ) ; } _lastKnownUserIds . add ( id . toString ( ) ) ; }
add a user id to the set of last known user ids
