Combine two markups in a toplevel markup

[Urs Liska]

Hi,

basically this is a follow-up question to the one where I needed to 
align a text over a slur. I wanted to apply the techniques I learned 
from your valuable help but ran against the next wall (or: I didn't see 
that there is no wall after all ...).

The overall goal now is to create a centered score with a centered 
caption below it. Of course I don't know how wide both the score and the 
caption will be.

My first attempt was quite straightforward: produce both the score and 
the markup as markups centered with \fill-line.

While this worked perfectly at aligning the score and markup it turned 
out not to be good enough: While this centers perfectly on the page I 
can't get correctly cropped output because the bounding box seems to 
reach both line ends, resulting in uncropped whitespace to the left and 
right of the score/caption.

If there's a solution to this problem everything below can be ignored.

So I thought to make this *one* markup stencil, align the two parts 
manually against each other and print the result as one (left-aligned) 
markup, so it should be properly cropped. Now while I feel quite 
confident that I would manage to deal with the alignment and stencil 
issues this time I don't see how I can get at them at the right moment 
when I'm dealing with a top-level markup (and not inside some music).

What I *have* is two markups (one regular and one score wrapped in a 
markup), both stored in variables.

What I *need* is a way to combine these markups to one and use the 
result as a top-level markup, the problem being that I need to access 
the resulting extents of the markups in order to properly align them 
against each other.

Any suggestions? (In fact I'd actually prefer suggestions over solutions 
this time ...)

Best
Urs