Editing Using header fields in arbitrary markup

This snippet adds a markup function that allows one to use header fields (e.g. the title, composer, etc.) in arbitrary markup. By default, markups like bookTitleMarkup specified in the \paper block is treated differently than other markup, so that one can access header fields via <code>\fromproperty #'header:fieldname</code> in such markups. This does not work for toplevel markup or markup inside a score, so <code>\fromproperty</code> returns an empty string in this case.<br />
The <code>\markupWithHeader</code> markup function defined in this snippet fixes this problem and makes the header fields available to markups, so that you can now use the same markups as for book/score titles also in other places throughout your score.

<lilypond version="2.24.0" full>
%% http://lsr.di.unimi.it/LSR/Item?id=467

% This function is originally copied from mark-up-title (file scm/titling.scm), 
% which is lilypond's internal function to handle the title markups. I needed 
% to replace the scopes and manually add the $defaultheader (which is internally 
% done in paper-book.cc before calling mark-up-title. Also, I don't extract the 
% markup from the header block, but use the given markup.
%
% I'm not sure if I really need the page properties in props, too... But I 
% suppose it does not hurt, either.
#(define-markup-command (markupWithHeader layout props markup) (markup?)
   "Interpret the given markup with the header fields added to the props.
This way, one can re-use the same functions (using fromproperty 
#'header:field) in the header block and as top-level markup."
   (let* (
	  ;; TODO: If we are inside a score, add the score's local header block, too!
	  ;; Currently, I only use the global header block, stored in $defaultheader
	  (scopes (list $defaultheader))
	  (alists (map ly:module->alist scopes))
	  (prefixed-alist
	   (map (lambda (alist)
		  (map (lambda (entry)
			 (cons
			  (string->symbol (string-append "header:"
							 (symbol->string
							  (car entry))))
			  (cdr entry)))
		       alist))
		alists))
	  (props (append prefixed-alist
			 props
			 (layout-extract-page-properties layout))))
     (interpret-markup layout props markup)))

% ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

% test markup containing references to header fields:

\paper { tagline = ##f }

testMarkup = \markup {
  \justify {
    This is a text inserted using the new \typewriter{"\\markupWithHeader"} 
    command to allow using header fields 
    (via "\fromproperty #'header:fieldname"). For example, the title field 
    is \fontsize #+3 \fromproperty #'header:title and 
    the composer is \fontsize #+3 \fromproperty #'header:composer
  }
}

\header { 
  title = "My own title" 
  composer = "Someone" 
  piece = "Global piece name"
}

% Print out the markups. The \fromproperty #'header:field are now properly
% interpreted.
\markup\markupWithHeader \testMarkup

\markup\markupWithHeader {
  \wordwrap {
    This is a long text, which is now able to use the header fields 
    (via "\fromproperty #'header:fieldname"). For example, the title field 
    of this score is: \fromproperty #'header:title and the composer is: 
    \fromproperty #'header:composer
  }
}
</lilypond>

[[Category:Text]]
[[Category:Scheme]]
[[Category:Titles]]