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References: <YW0u7vN+Y/604YuY@polynum.com>
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Message-ID:  <7777989c-8044-08a3-3a9d-2da8ec591a2b@morningstar2.co.uk>
Date:         Mon, 18 Oct 2021 14:04:20 +0100
Reply-To:     Mailing list for the LaTeX3 project
              <LATEX-L@LISTSERV.UNI-HEIDELBERG.DE>
Sender:       Mailing list for the LaTeX3 project <LATEX-L@LISTSERV.UNI-HEIDELBERG.DE>
From:         Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: expl3-code and Prote
To:           LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
In-Reply-To:  <YW0u7vN+Y/604YuY@polynum.com>
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Envelope-To: <rainer.schoepf@gmx.net>
Status: R

Hello Thierry

>  From an engineering point of view, couldn't the code set everything with
> some:
> 
> \ifx\engineversion\undefined
> \else
>    \input engine.tex
> \fi
> 
> ? So that it's easier to see what depends on the engine and in order
> for modifications to the engine to be severed from the rest of the code?

Very little of the code is engine-specific, and the parts that are are 
largely in separate files as you suggest. Broadly

- l3names needs to save primitives in a predictable way, so checks for
   example for pdfTeX and LuaTeX names for the same concepts
- l3sys sets up engine-specific paths
- l3backend is split by backend: you likely are using dvips so
   l3backend-dvips.def needs to be loaded

I see Phelype has addressed the other parts.

Joseph