diff --git a/collects/typed-racket/optimizer/tool/mzc.rkt b/collects/typed-racket/optimizer/tool/mzc.rkt
index b4fb0b7d0a..b6cf7c0075 100644
--- a/collects/typed-racket/optimizer/tool/mzc.rkt
+++ b/collects/typed-racket/optimizer/tool/mzc.rkt
@@ -147,9 +147,7 @@
   (define-values (inliner-logs tr-logs)
     (partition inliner-log-entry? log))
   (define grouped-events
-    (group-by (lambda (x y)
-                (equal? (log-entry-pos x) ; right file, so that's enough
-                        (log-entry-pos y)))
+    (group-by equal? #:key log-entry-pos ; right file, so that's enough
               inliner-logs))
   (define new-inline-log-entries
     (for*/list ([g     (in-list  grouped-events)]
diff --git a/collects/typed-racket/optimizer/tool/utilities.rkt b/collects/typed-racket/optimizer/tool/utilities.rkt
index 7b85becf33..242ab3ad8a 100644
--- a/collects/typed-racket/optimizer/tool/utilities.rkt
+++ b/collects/typed-racket/optimizer/tool/utilities.rkt
@@ -8,10 +8,10 @@
   (for/list ([l (in-list log)] #:when (regexp-match r l))
     l))
 
-;; (x x -> bool) (listof x) -> (listof (listof x))
+;; (y y -> bool) (listof x) #:key (x -> y) -> (listof (listof x))
 ;; groups together elements that are considered equal
 ;; =? should be reflexive, transitive and commutative
-(define (group-by =? l)
+(define (group-by =? l #:key [key values])
   (for/fold ([res '()]) ; list of lists
       ([elt (in-list l)])
     (let loop ([classes     res] ; "zipper" of the equivalence classes
@@ -19,7 +19,7 @@
       (cond [(null? classes)
              ;; did not find an equivalence class, create a new one
              (cons (list elt) res)]
-            [(=? elt (car (car classes)))
+            [(=? (key elt) (key (car (car classes))))
              ;; found the equivalence class
              (append rev-classes ; we keep what we skipped
                      ;; we extend the current class
@@ -31,4 +31,3 @@
 ;; TODO add to unstable/list, and add tests. here's one
 ;; -> (group-by = '(1 2 1 2 54 2 5 43 7 2 643 1 2 0))
 ;; '((0) (2 2 2 2 2) (7) (43) (5) (54) (643) (1 1 1))
-;; TODO needs a #:key arg