>> Could you please try to instrument the function `find-source-lisp-file'
>> (in help-fns.el) for debugging before you do C-h f and tell me what it
>> finds?
>
> `describe-simplify-lib-file-name' already converts "/Users/mt/prolog.el"
> to "prolog.el", which I think it should not do to preserve the origin.
> `find-source-lisp-file' then expands it to the Emacs source file.
Maybe. Any ideas how to do that without breaking the rest of the code?
IIUC the problem is with `find-source-lisp-file' which searches for an
`elc-file' that does not exist for _your_ prolog.el, finds the compiled,
built-in one instead, ...