seq-sort-by unavailable in Emacs 25, so spell it out

racket-describe.el

@@ -712,7 +712,7 @@ as invisible text."
           ((string-equal "" from)
            (make-string 16 ?z))
           (t from)))
-       (sort-key (v)
+       (key (v)
          (pcase-let
              ((`(,what ,from ,fams) (get-text-property 0 'racket-affix v))
               (sort (get-text-property 0 'racket-sort v)))
@@ -728,10 +728,14 @@ as invisible text."
        (key< (as bs)
          (cl-loop for a in as
                   for b in bs
-                  unless (generic= a b)
-                  return (generic< a b)
+                  unless (generic= a b) return (generic< a b)
                   finally return nil)))
-    (seq-sort-by #'sort-key #'key< strs)))
+    ;; `seq-sort-by' unavailable in Emacs 25, so instead of
+    ;; (seq-sort-by #'key #'key< strs) spell it out:
+    (seq-sort (lambda (a b)
+                (key< (key a)
+                      (key b)))
+              strs)))
 
 (provide 'racket-describe)