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On Thu, Jan 22, 2009 at 08:52:43PM +0000, Joseph Wright wrote:

> Will Robertson wrote:
> > From the way things look to me, that will basically be impossible. The
> > changes between LaTeX3 and LaTeX2e are much greater than between 2e and
> > 2.09. Perhaps a strict subset of LaTeX2e documents will be able to be
> > processed with LaTeX3, but I think we'll end up with two distinct
> > document processing systems. That is, I doubt that LaTeX2e will ever be
> > "replaced" by LaTeX3 -- people who need backwards compatibility will
> > have to use the old code.
> > 
> > On the other hand, it might be possible to detect when a 2e document is
> > being processed and "drop down" into the old code automatically, but
> > it's probably too early to say since we're not even building a separate
> > LaTeX3 format at this stage (well, I'm not, at least).
> 
> I'd imagine that LaTeX3 documents will start with something other than
> \documentclass (let's call it \documenttype, for arguments sake).  Given
> the likely size of a LaTeX3 kernel, essentially the entire 2e kernel
> could also be included without making it all that much bigger.  If the
> first line of the file is \documenttype, the 2e stuff is never used.  On
> the other hand, if \documentclass is found, the new kernel "bails out",
> runs the current 2e code and the file is processed as a 2e document.

In theory. In practice, you can or sometimes even must have some stuff
before \documentclass/\documenttype.
Example:
\RequirePackage{cmap}% before the class loads fonts
\documentclass{myclass}

Yours sincerely
  Heiko <[log in to unmask]>