diff --git a/src/11.rkt b/src/11.rkt
index 865f614..ebbf479 100644
--- a/src/11.rkt
+++ b/src/11.rkt
@@ -1,6 +1,7 @@
 #lang curly-fn racket
 
-(require "../lib.rkt")
+(require racket/set
+         "../lib.rkt")
 
 (define input
   (for/vector ([row (problem-input 11)])
@@ -15,7 +16,11 @@
   (vector-ref (vector-ref grid row) col))
 
 (define (grid-update! grid row col f)
-  (vector-set! (vector-ref grid row) col (f (grid-ref grid row col))))
+  (let ([vec (vector-ref grid row)])
+    (vector-set! vec col (f (vector-ref vec col)))))
+
+(define (grid-set! grid row col v)
+  (grid-update! grid row col (const v)))
 
 (define (adjs grid row col)
   (for*/list ([r (range* (max 0 (sub1 row)) (min (sub1 rows) (add1 row)))]
@@ -23,35 +28,35 @@
     (cons r c)))
 
 (define (incr! octopodes)
-  (for ([row (range rows)])
-    (for ([col (range cols)])
-      (grid-update! octopodes row col add1))))
+  (for* ([row (range rows)]
+         [col (range cols)])
+    (grid-update! octopodes row col add1)))
 
 (define (flash! flashed octopodes)
-  (for*/fold ([newly-flashed '()])
+  (for*/fold ([newly-flashed (set)])
              ([row (range rows)]
               [col (range cols)])
-    (if (and (not (member (cons row col) flashed))
+    (if (and (not (set-member? flashed (cons row col)))
              (> (grid-ref octopodes row col) 9)
              (for ([adj (adjs octopodes row col)])
                (grid-update! octopodes (car adj) (cdr adj) add1)))
-        (cons (cons row col) newly-flashed)
+        (set-add newly-flashed (cons row col))
         newly-flashed)))
 
 (define (decharge! octopodes)
-  (for ([row (range rows)])
-    (for ([col (range cols)])
-      (when (> (grid-ref octopodes row col) 9)
-        (grid-update! octopodes row col (const 0))))))
+  (for* ([row (range rows)]
+         [col (range cols)])
+    (when (> (grid-ref octopodes row col) 9)
+      (grid-set! octopodes row col 0))))
 
 (define (step! octopodes)
   (incr! octopodes)
   (define flashes
-    (let loop ([flashed '()])
+    (let loop ([flashed (set)])
       (let ([newly-flashed (flash! flashed octopodes)])
-        (if (empty? newly-flashed)
-            (length flashed)
-            (loop (append newly-flashed flashed))))))
+        (if (set-empty? newly-flashed)
+            (set-count flashed)
+            (loop (set-union newly-flashed flashed))))))
   (decharge! octopodes)
   flashes)
 
@@ -60,9 +65,8 @@
     (step! input)))
 
 (define part2
-  (let loop ([steps 101])
-    (if (= (step! input) total)
-        steps
-        (loop (add1 steps)))))
+  (for/last ([steps (in-naturals 101)]
+             #:final (= (step! input) total))
+    steps))
 
 (show-solution part1 part2)
\ No newline at end of file