> You can use (elpa-directory #$output) instead of (string-append ...)

   Thanks, I did not know about elpa-directory.

   > You can use for-each instead of map since you don't need the return
   > value. Moreover, `copy-recursively' is probably shorter.
   >
   > However, this will catch neither the ".el" file nor the ".lsp" file 
in
   > "implementations/easy-islisp" directory. What should we do about the
   > latter ?

   It seems Emacs has a hard time finding easy-islisp.el when using
   copy-recursively, since it's put inside a directory. The easy solution
   would be to 'expand' the regex in find-files like this:

   (lambda _
     (let ((site-lisp-dir (elpa-directory #$output)))
       (for-each (lambda (dir)
		  (copy-recursively dir elpa-directory)
		  (with-directory-excursion dir
		    (for-each (lambda (file)
				(install-file file
					      site-lisp-dir))
			      (find-files "." "\\.(lsp|el)$"))))
		'("advance" "implementations"))))

   Are there any objections to doing it this way? (Other than it being a
   lot less pretty than a call to copy-recursively 😄)

   > You also need to add `eisl' as an input,
   > and patch "easy-islisp.el"
   > accordingly (see `easy-islisp-executable').

   I incorporated this in the new patch.

   I have attached an updated version below.

   Thanks for reviewing!