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Date:         Sat, 22 Aug 2015 08:37:09 +0100
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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: Help with Expl3-izing some code
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On 21/08/2015 18:16, Joel C. Salomon wrote:
> Experimentation, and looking at xbox code, shows me that
> `\hbox_overlap_left:n` can substitute for `\llap`. I’m not sure why
> l3trial/xbox’s `\llap` includes `\xbox_leave_vmode:`, though—probably
> for the same reason the Memoir example code included `\leavevmode`,
> but I don’t understand that either; it seems to work just fine without
> it.

I see you've had some other answers that cover a lot of what I'd say :-)
To add a few notes, the main reason I wrote xbox was to see how well
xparse/expl3 can cope with this type of higher-level stuff. Thus it's
mainly about experiments rather than expecting the code as-is to be used
in production.

As Chris notes \llap, etc. are really plain TeX commands. The *don't*
current do \leavevmode so will fail ('not behave nicely') if used when
TeX is in vertical mode. All of the LaTeX box commands do \leavevmode so
it seemed to me that a new implementation should alter this.
--
Joseph Wright