Comment 12 for bug 1875475

On Ubuntu 20.04, I successfully downgraded Unison (text only) to the version compiled with OCaml 4.02 from https://www.urs-mueller.ch/de/computer/download/unison-2.48.4-linux-x86_64-text-static.tar.gz

Using the unison-jz snap installing Unison 2.51.2 compiled against OCaml 4.02.3 as described by https://unix.stackexchange.com/questions/583058/unison-and-version-compiler-conflicts/583377#583377 requires that :
- The --devmode option of the snap command is NOT necessary,
- The problem of the Include line in /etc/ssh/ssh_config is solved,
- Binaries of Unison 2.51.2 compiled against OCaml 4.02.3 are also provided for :
  - Mac OS-X, for example at https://www.urs-mueller.ch/de/computer/index.html
  - Windows, for example at https://www.irif.fr/~vouillon/unison/

On the long term, I would prefer that the Unison packages explicitly contain the OCaml version, and the default value of servercmd should contain the name of the server binary compiled with the same OCaml version. For example :

- unison-OCaml-4.02 version 2.48.4 should contain
  /usr/bin/unison-OCaml-4.02 --> /usr/bin/unison-OCaml-4.02-2.48.4

- unison-OCaml-4.02 version 2.51.2 should contain
  /usr/bin/unison-OCaml-4.02 --> /usr/bin/unison-OCaml-4.02-2.51.2

- unison-OCaml-4.06 version 2.48.4 should contain
  /usr/bin/unison-OCaml-4.06 --> /usr/bin/unison-OCaml-4.06-2.48.4

- unison-OCaml-4.06 version 2.51.2 should contain
  /usr/bin/unison-OCaml-4.06 --> /usr/bin/unison-OCaml-4.06-2.51.2

  This would permit, on the server, to install Unison binaries compiled with ALL OCaml versions.