Merging paper blocks

[Urs Liska]

Hi all,

some time ago I got assistance for merging layout blocks that were 
stored in a list. The code I used (and which works, although I don't 
fully understand it) is something like this:

    ((layout-diffs
      (append-map
       (lambda (output-def)
         (lset-difference
          equal?
          (ly:output-find-context-def output-def)
          (ly:output-find-context-def $defaultlayout)))
       (a-list-holding-multiple-layout-expressions)))

    ;; apply `diff' to the default layout
    (for-each
     (lambda (x)
       (ly:output-def-set-variable! $defaultlayout (car x) (cdr x)))
     layout-diffs)

(I understand the concept of stripping the diffs from the layout 
expressions against the default layout and then applying each 
modification in turn. What I don't understand is that lupon inspection 
'layout-diffs' seems to hold one expression for each *context 
modification* that isn't present in the default layout. So in my case 
there is one item for FiguredBass (which I understand), but no one for 
e.g. Voice or Staff (which I don't understand because there *are* 
corresponding entries in the layout blocks.)

Is there a similar approach to merge \paper {} blocks? I need this to 
create hierarchical styles (no, not for GSoC, but the experience should 
be transferable).
When a \paper {} block is defined in a "common" file and then again in a 
"special" file (e.g. for "score" or "part") the later simply replaces 
the former.
What I need is a way to store a \paper block in a variable and then 
update it with individual settings from other paper blocks.

Any pointers would be appreciated!
Urs