static DefaultMutableTreeNode jDynamicTree_addToNode(final JTree tree, fO node, fO child) {
  ret swing(func -> DefaultMutableTreeNode {
    DefaultMutableTreeNode n = jDynamicTree_findNode(tree, node);
    if (n == null || jDynamicTree_isInDummyState(n)) null;
    DefaultMutableTreeNode childNode = jDynamicTree_newNode(child);
    n.add(childNode);
    ((DefaultTreeModel) tree.getModel()).nodesWereInserted(n, new int[] { n.getChildCount()-1 });
    ret childNode;
  });
}