Incipit

[Andreas Schneider]

I'm trying to write a Scheme script for an incipit
(https://lilypond.org/doc/v2.23/Documentation/notation/working-with-ancient-music-_002d-scenarios-and-solutions#incipits).
I have started from the script for instrument name and modified it to
match my needs arriving at the following:

;;;Incipit
(let ((tag  "Incipit") (current "") )
(if (string? Incipit::params)
  (begin
    (set! current Incipit::params)
    (set! Incipit::params #f)))
(set! current (d-GetUserInput (_ "Incipit") (_ "Give incipit for current
staff:") current))
(if (string? current)
  (begin
  (d-DirectivePut-staff-prefix tag  (string-append "incipit { " current
" }")
  (set! Incipit::params current)))))

Unfortunately, I get a script error that I do not understand. Any idea
what is wrong?

Thanks in advance

Andreas