markup->string and user markup commands

[Jérôme Plût]

#(begin
; after http://lsr.di.unimi.it/LSR/Snippet?id=969
(define-markup-command (with-background layout props color arg) (color? markup?)
   (let* ((stencil (interpret-markup layout props arg))
          (X-ext (ly:stencil-extent stencil X))
          (Y-ext (ly:stencil-extent stencil Y)))
     (ly:stencil-add (ly:make-stencil
                      (list 'color color
                        (ly:stencil-expr (ly:round-filled-box X-ext Y-ext 0))
                        X-ext Y-ext)) stencil)))
(display "\"")
(display (markup->string (markup #:with-background `(1. 0. 0.) "something")))
(display "\"")
)

This should display "something". However it displays an empty string.
How could I fix this?

(I tried to manually debug markup->string (in scm/markup.scm): as I
understand it, the problem is likely that with-background does not
belong to the 'lily module. However, I am completely unfamiliar with
guile's module system, so I don't have the slightest idea about how to
fix this).

Thanks,

-- 
        Jérôme