public void add(TwoDNode dn){
  if (inProgress) {
    throw new ConcurrentModificationException("Can't add point while iterator in progress");
  }
  nodes.add(dn);
}

public void add ( TwoDNode dn ) { if ( inProgress ) { throw new ConcurrentModificationException ( "Can't add point while iterator in progress" ) ; } nodes . add ( dn ) ; }
add subtree rooted at given node to the result set .
