Re: Major modes using `widen' is a good, even essential, programming practice.

[Gregory Heytings]

> > It is wrong that "arbitrary Lisp can be executed through 
> > fontification-functions", as you said earlier.
>
> It is not wrong. [...]
>
> > A function called from fontification-functions isn't supposed to 
> > download a file, or to send an email, or to change a user option, or to 
> > remove or create a file, or to remove or insert text in the buffer, or 
> > to kill Emacs or a frame or a window or the current buffer, or to 
> > change the window layout, and so on and so forth.
>
> That doesn't deserve a reply, so won't be getting one.

Yet the above are all perfectly legitimate examples or "arbitrary Lisp". 
So I take it that in fact you agree that it is wrong that "arbitrary Lisp 
can be executed through fontification-functions".  Neither the 
deliberately extreme examples above, nor anything else that is outside of 
the scope of the API contract.  Code executed through 
fontification-functions should do what it was designed to do, and only 
that, otherwise it breaks the API contract.