Pasting together markup expressions

[David Kastrup]

Ok, the following somewhat naive approach which treats music and markup
expressions sort of like cut&pasteable strings falls apart horribly.

How do I do this sort of thing properly?

The goal is to be able to write something like

\Discant #130

and get an accordion discant symbol with 1 dot at the top, 3 in the
middle, and none in the bottom.  There is a slight twist where the
digits 4 5 6 7 in the middle place deliver asymmetric one- and two-dot
patterns (those are sometimes used to indicate different configurations
on special instruments).  But that just pertains to the right-dot
formula and is not really relevant for the problem.

discant = \markup {
   \musicglyph #"accordion.accDiscant"
}
dot = \markup {
   \musicglyph #"accordion.accDot"
}
Discant = #(define-music-function (parser location code) (number?)
            (let* ((piccolo (if (zero? (modulo code 100)) #{ #}
                            #{ \combine \raise #2.5 \dot #}))
             (left (if (even? (quotient code 20)) #{ #}
                    #{ \combine \translate #'(-1 . 0) \raise #1.5 \dot #}))
             (middle (if (even? (quotient code 10)) #{ #}
                      #{ \combine \raise #1.5 \dot #}))
             (right (if (eq? (even? (quotient code 20))
                         (even? (quotient code 40))) #{ #}
                     #{ \combine \translate #'(1 . 0) \raise #1.5 \dot #}))
             (bass (if (even? code) #{ #}
                    #{ \combine \raise #0.5 \dot #})))
            #{ ^\markup { $piccolo $left $middle $right $bass discant } #}))

\layout { ragged-right = ##t }

\score { \relative c' { c\Discant #131 d \Discant #11 }}


-- 
David Kastrup