Last night, I wrote:
> What is the cleanest way to do this with Expl3? Specifically, what are
> the Expl3 equivalents of \parbox and \llap, at least in the way
> this code uses them?
>
> (I understand that \raggedleft would be in a very different layer of
> the code [the galley?] and probably doesn’t have a useable Expl3
> equivalent yet; and that using \tex_kern:D is no clearer than
> \kern, and that its Expl3ization will probably be in the fonts
> module. On the other hand, I see that l3trial/xfont/xfss.dtx includes
> a definition for \mode_leave_vertical: so perhaps I’ll copy its
> definition and use that.)

Hunting around the latex3 repository it seems I’m anticipating what
l3trial/xbox is doing. (It’s not stable, I understand, but is xbox in
a somewhat usable state if I take it from the repository? I’m quite
willing to change my [private] code as xbox updates, assuming the
current state at least broadly approximates future plans.)

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.

The commit message at
<https://github.com/latex3/latex3/commit/432d3d9> indicates that xbox
does not yet have the equivalent of \parbox. For the restricted use
I’m putting this to (\parbox[t][0pt]{\somedim}{one or two lines of
text}), can somebody suggest a lightweight alternative? Neither
wrapping \hbox_to_wd:nn within \vbox_to_zero:n nor the other way
around worked for me.

—Joel C. Salomon