diff --git a/prospect/examples/bank-account.rkt b/prospect/examples/bank-account.rkt
index af94ac6..c16e4da 100644
--- a/prospect/examples/bank-account.rkt
+++ b/prospect/examples/bank-account.rkt
@@ -13,7 +13,7 @@
 
 (define (observer e _)
   (when (patch? e)
-    (for [(balance (project-assertions (patch-added e) (account (?!))))]
+    (for-trie ([(account $balance) (patch-added e)])
       (printf "Balance changed to ~a\n" balance)))
   #f)