Kernel WYSIWYG rooted digraphs

Previously

I said in comments that I view backquote as a WYSIWYG tree ctor, analogous to list'.

That's not fully general

But backquote doesn't suffice to construct the most general form of open structure of pairs, a rooted digraph. Those require the ability to share objects between multiple parents.

Things that don't do the job

One could write shared objects by naming objects in a let' form and including them by name. That's not WYSIWYG, though. WYSIWYG would at least present the objects in place once, as #N# / #N= reader notation does.

But that's in the reader. It's not available to general code.

What to do

One could:

• Enclose the whole construction in an empty let'.
• Provide a combiner expanding to ($sequence ($define! x DEF) x)`