public void plus(Closure c){
  if (parent() == null) {
    throw new UnsupportedOperationException("Adding sibling nodes to the root node is not supported");
  }
  appendNodes(c);
}

public void plus ( Closure c ) { if ( parent ( ) == null ) { throw new UnsupportedOperationException ( "Adding sibling nodes to the root node is not supported" ) ; } appendNodes ( c ) ; }
adds sibling nodes ( defined using builder - style notation via a closure ) after the current node .
