Ok ... So, some precisions after some tests (and losing my last hair 
) :
In fact ... list-ref doesn't work ... because gimp-image-get-layers doesn't return a list for the layer IDs ...
gimp-image-get-layers is said to return "The number of layers contained in the image (num-layers >= 0) [INT32]" and "The list of layers contained in the image. [INT32ARRAY]".
So, as "gimp-..." procedures always return a list, I thought gimp-image-get-layers returns a list with two elements : 1- the number of layers (a number) ; 2- a list of the layers' IDs (a list). But it doesn't work with this assessment...
I experimented a lot, and the list returned is indeed a two-element list, with the first term indeed being a number = the number of layers (ok, and easy to get --> "car (gimp-image-get-layers Image)") ; but the second element is not a list ... but a vector ! (Spent hours to find that ...
Ok the description talks about "INT32ARRAY", maybe a term to mean "vector" ... but it's not specified ... and description talks about "list of layers" ...)
Definitely bald now, but it finally works, using vector-ref !
(can get easily the IDs' list (as a vector...) with "cadr (gimp-image-get-layers Image)").
((Could be usefull if someone else is asking.))
) :In fact ... list-ref doesn't work ... because gimp-image-get-layers doesn't return a list for the layer IDs ...
gimp-image-get-layers is said to return "The number of layers contained in the image (num-layers >= 0) [INT32]" and "The list of layers contained in the image. [INT32ARRAY]".
So, as "gimp-..." procedures always return a list, I thought gimp-image-get-layers returns a list with two elements : 1- the number of layers (a number) ; 2- a list of the layers' IDs (a list). But it doesn't work with this assessment...
I experimented a lot, and the list returned is indeed a two-element list, with the first term indeed being a number = the number of layers (ok, and easy to get --> "car (gimp-image-get-layers Image)") ; but the second element is not a list ... but a vector ! (Spent hours to find that ...
Ok the description talks about "INT32ARRAY", maybe a term to mean "vector" ... but it's not specified ... and description talks about "list of layers" ...)Definitely bald now, but it finally works, using vector-ref !
(can get easily the IDs' list (as a vector...) with "cadr (gimp-image-get-layers Image)").
((Could be usefull if someone else is asking.))