Thanks for quick answers, Jean and David! The function "ly:music-deep-copy" is the solution I was looking for :-)
> module-ref and module-set! are useful for dealing with variables of which the name is not known in advance.
Actually, in my case I don't know in advance the name of a variable which value I want to copy, that's why I used the module-ref function. In the example presented to you I used a symbol, because I wanted to keep the example as simple as possible.
> It assumes that the #{ c' des' #} part comes from something programmatically,
Yes, that's the reason.
> The other way is to use ly:music-deep-copy and it's not like this isn't mentioned in the "LilyPond — Extending" guide of LilyPond
You know, David, I just got lost. I wish I had more time to work with my project.
Many many thanks!
Robert