My Emacs mode already uses the external function editor. You access it using C-c C-f. What I did was also to detect when ∇ is typed, and intercept that command to also open up the function editor that way. Thus, right now, there are two ways to open the function editor in Emacs.
The reason I did this was that the use of ∇ is probably much more well known than having to figure out that you can press C-c C-f to achieve the same effect.
The reason for my post above was to get a better way to do this hook into the ∇ command, as the method I'm using right now has some reliability issues.
That said, removal of the ∇ command altogether would, of course, remove the need for this callback mechanism.
Regards,
Elias