Re: installing two versions of a user application

[Stefan Monnier <monnier@iro.umontreal.ca>]

> You probably want to look inside the control archives rather than the
> data archives:
>
> https://en.wikipedia.org/wiki/Deb_%28file_format%29
>
> If both control archive files are using a package name of
> "master-pdf-editor", you can try Stefan's suggestion of renaming one
> and seeing if they'll then coinstall.

Exactly: the immediate problem you're encountering is that the package
names themselves (regardless of the files they contain) collide.
This info is the `control` part of the .deb archive.

The way I do it is to use Emacs to open the `foo.deb` file.
This should show you something like:

    M    Mode      Size      Date&time       Filename
    - ----------  ------  ----------------  ----------------
      -rw-r--r--       4  2021-03-01 12:23  debian-binary
      -rw-r--r--    1128  2021-03-01 12:23  control.tar.xz
      -rw-r--r--  615380  2021-03-01 12:23  data.tar.xz
    - ----------  ------  ----------------  ----------------
                  616512                     3 files

where the `data.tar.xz` part contains the actual files and
`control.tar.xz` is the part you want to modify.
So you select the `control.tar.xz` which itself will look somewhat
similar and will contain a `control` which you can open and in which
you'll find presumably something like:

    Package: master-pdf-editor
    [...]

followed by other kinds of metainfo about the package.  Just edit that
first line (e.g. by adding a `4` or `5`), then do `C-x C-s` to save this
`control` back into `control.tar.xz`, then kill this buffer with `C-x
k RET`, then save the new `control.tar.xz` back into the actual foo.deb
file with `C-x C-s`.

Then retry to install those debs with:

    dpkg --install foo.deb

I recommend you do those modifications on a copy of the file, just so
its easier to recover from mistakes.


        Stefan