Dear Ihor,

> Another use case for me is to speed up agenda creation.
> I usually do not like to split my org files into too many. However, it
> results in very large and slow org buffers later. If I can store some
> parts of the org files externally and only show them if some condition
> is met (say, for certain todo state of the parent entry), it would speed
> up my agenda and the buffer navigation quite significantly.

That's a good one!

> Let me put some historical context to this proposal.
> There was a discussion of similar feature in emacs-dev last year.
> The idea was to implement nested buffers:
> https://lists.gnu.org/archive/html/emacs-devel/2018-07/msg00863.html

An interesting read, provides another use-case (collect external data in
one place to easily view/edit):
https://lists.gnu.org/archive/html/emacs-devel/2018-07/msg00890.html

> There are also several projects, which implement part of the
> functionality you described:
> - mmm-mode: https://github.com/purcell/mmm-mode
> - polymode: https://github.com/polymode/polymode

Pretty cool stuff. For thoroughness, let's discuss how these work.

I found a comment which mentions polymode's working principle.
https://www.reddit.com/r/emacs/comments/50p34n/polymode_is_awesome/?depth=1
>> Polymode doesn't keep its modes in a single emacs buffer but in several
indirect buffers, as many as different modes are there in a file.
Consequently, polymode is as fast as switching emacs buffers because it
never re-installs major modes like other multi-modes do. Dave Love's
multi-mode.el gets full credit for this idea.
> It looks like it slows emacs to a crawl in my main org config file. It
seems to work fairly well in some of my notes files (though with some weird
indenting behavior).

Basically, simplicity is in place but at the cost of duplication.
Lenses could avoid duplication, while yielding increased functionality and
speed.
(e.g. in polymode, a syntax checker couldn't yield correct results unless
narrowing was constantly used, which is inefficient)

Now, to MMM-mode. According to the info file:

> Within the file, MMM-mode creates /submode regions/ within which other
major modes are in effect.

> While the point is in a submode region, the following changes occur:
> <...> keymap <...> local variables <...> syntax table and indentation
<...> font-lock

> The submode regions are represented internally by Emacs Lisp objects
known as /overlays/.

> A lot of the functionality of MMM Mode---that which makes the major mode
> appear to change---is implemented by saving and restoring the values of
> local variables, or pseudo-variables.

What I don't understand is where the modes of the submode region run and
when they are turned on.
Are necessary modes just allowed to run at the right time for the whole
buffer? But then, how are they limited in their effect to just the
necessary region? Narrowing?
Could, for example, syntax checking be done efficiently that way?
Could someone, please, explain?

Best regards,
Dmitrii.