[JAVA3D] Editing live scene graphs

[Shantz Family]

I'm doing dynamics and need to edit a live scene graph.  With a live scene graph like this      -> BG->BG1->TG->L->SG->Shape                            ...                      ->TG->L--^   (BG=BranchGroup, TG=TransformGroup, L=Link, SG=SharedGroup)   I can successfully add new instances of Shape by detaching BG1, adding a new TG->L to BG1, and reattaching BG1.  That works fine.   So I thought that if I detached a BG I could add stuff to it but that doesn't work because even after being detached the BG subtree complains that "only BGs may be added".    So how does one edit a detached BG subtree that was once live?  Do you have to make a new BG and copy the whole thing?    Or perhaps someone can point me to a book that has more to say about editing live scene graphs.  The books (Selman, j3d spec) don't say much about editing live scene graphs.   Mike