Earlier quoted context omitted.
I came here to post this. To expand: With Leo editor, you convert the document/file into a tree of nodes (one way to do this is to make each function a node - they have plugins to do it automatically for well known languages like C++). Let's say you make a particular function a node. You can then make a new document in your own filesystem which has your notes, but you can make a "live" copy of the node linking to tha…
> It's the one powerful feature that has yet to be replicated in Emacs. Emacs cannot do this. There are many Emacs libraries or packages that need this feature (Org babel is a big one, transclusion is another), and have to work around its absence in hacky ways.
I don't know if there is any fundamental limitation in Emacs/Elisp that prevents it, or that no one has succeeded in doing it. I suspect it's the latter. I also suspect the problem is trying to shoehorn this to work with Org Mode (which is what I want as well), but there may be a significant impedance mismatch between the Org code base and this feature.
Frankly, trying to manipulate the Org tree using Elisp is a nightmare compared to how simple it is in Leo (using Python). I've been trying to do with Org mode what is fairly basic in Leo: Traverse the tree, make changes, copy the tree to another file, with some headlines demoted/promoted due to rules, etc. Although I finally got something working, it took a lot of research as well as multiple packages. Whereas in Leo, people without a programming background manage to write the Python code to do this very easily.
[1] karthink has written a number of heavily used Emacs packages. He likely knows what he's talking about.