Dear Dmitrii,

I strongly support the proposal.

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.

Example:
#+begin_src org
* Projects
** 2019
*** TODO Project 1     :ORG:
# the project contents is stored in an external file
:PROPERTIES:
:ORG-FILE: project1.org
:END:
# beginning of a lense, which is linked to project1.org
**** Heading 1
**** Heading 2
And many headings below
# ...
# end of the lense
*** HOLD Project 2     :ORG:
:PROPERTIES:
:ORG-FILE: project2.org
:END:
# beginning of another lense
# nothing is included here because the project state is =HOLD=
# end of the lense
#+end_src

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 

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

Best,
Ihor

Dmitrii Korobeinikov <dim1212k@gmail.com> writes:

> I have written a proposal for buffer lenses which could prove useful in
> Org-mode, especially for interacting with code.
> If you are interested, please, see this link:
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=35419

-- 
Ihor Radchenko,