liststring = 
#(define-music-function (parser location listarg stringarg) 
((list? '()) string?) 
(let ((str stringarg))
#{ 
  c''1-\markup $str
#}))


listnumber = 
#(define-music-function (parser location listarg numberarg) 
((list? '()) number?) 
(let ((str (number->string numberarg)))
#{ 
  c''1-\markup $str
#}
))


{
  s1_"empty listarg supplied:"                            \break
  s1_"\liststring #'() \"aaa\"" \liststring #'() "aaa" s1 \break
  s1_"\listnumber #'() 111"     \listnumber #'() 111 s1   \break
}
{
  s1_"listarg not supplied:"                              \break
  s1_"\liststring \"bbb\""      \liststring "bbb" s1      \break % 2.16.2 ok   2.18.2 nok
  s1_"\liststring \"222\""      \liststring "222" s1      \break % 2.19.39 ok  2.19.80 nok
  s1_"\listnumber 333"          \listnumber 333 s1        \break % 2.19.38 ok  2.19.39 nok
}