Increment variable in Lilypond function

Question

My Lilypond score consist of repeat snippets a few bars each. I need to show the snippet number as markup. The score is being updated regularly by inserting new snippets, hence the snippet number should increment automatically, rather than being hard-coded:

snippetNumber = 0

snippet = #(define-music-function)
  (music)
  (ly:music?)
  #{
    % How do I increment the snippetNumber variable here?
    \textMark #(number->string snippetNumber)
    \repeat volta 2 {
      #music
    }
  #})

\snippet { c8 e fis e d fis gis fis }
\snippet { c a f cis d b g dis }

Answer 1

Add this where the comment is:

#(set! snippetNumber (+ 1 snippetNumber))

With the syntax correction, this gives

snippetNumber = 0

snippet = #(define-music-function
  (music)
  (ly:music?)
  #{
    #(set! snippetNumber (+ 1 snippetNumber))
    \textMark #(number->string snippetNumber)
    \repeat volta 2 {
      #music
    }
  #})

\snippet { c8 e fis e d fis gis fis }
\snippet { c a f cis d b g dis }