import javax.swing.tree.*;

// dummy text you can use as a temporary child for a lazy-filled node
sS jDynamicTree2_dummy = new String("<dummy>");

static <A> JTree jDynamicTree2(final A root, final F1<A, L<A>> makeChildren) {
  ret swing(func -> JTree {
    DefaultMutableTreeNode rootNode = new(root);
    rootNode.add(new DefaultMutableTreeNode(jDynamicTree2_dummy));
    final JTree tree = new JTree(rootNode);
    tree.setShowsRootHandles(true);
    tree.collapseRow(0);
    
    tree.addTreeWillExpandListener(new TreeWillExpandListener {
      public void treeWillCollapse(TreeExpansionEvent e) {}
      public void treeWillExpand(TreeExpansionEvent e) {
        DefaultMutableTreeNode node = cast e.getPath().getLastPathComponent();
        L<A> l = callF(makeChildren, (A) node.getUserObject());
        ret replaceTreeNodeChildren_withDummyChild(node, l, jDynamicTree2_dummy);
      }
    });
    
    ret tree;
  });
}