[xubuntu-users] Installing an older version of a package

[Peter Flynn]

I would like to be able to use an older version of Emacs (25.2.2) in a 
current distro of Mint (or some other Ubuntu or derivative). The current 
version of Emacs in the repos for (eg) Mint 20.2 is 26.3. The 25.2.2 
version of Emacs was last distributed for Mint 19.2 (maybe different for 
different distributions).

[Reason: 25.2.2 appears to be the last version of the Emacs deb to work 
with the psgml package that provided the authorial document-oriented 
xml-mode able to parse the DTD, before it was superseded by the less 
usable data-oriented xml-mode which only handles RNG.]

Where would I find the .deb that would have been installed by Mint 19.2 
in the days when that was current? I tried to work it out by booting 
19.2 from USB and installing Emacs but my interpretation of the 
arguments to archive.ubuntu.com (bionic, main, amd64, etc) doesn't seem 
to lead me to a downloadable .deb file.

It may of course refuse to install, believing that up-to-date versions 
of the libraries are not going to support its older exepctations. Fine.

I could try to build from source, but this is notoriously difficult, and 
doesn't mesh easily with the Ubuntu way of organising the filesystem.

I need to run Emacs in direct interaction with other software in the 
same session, so using a VM is not an option (and this machine is too 
slow anyway).

Peter