diff --git a/semantics.rkt b/semantics.rkt
index a6cf7c2..4b02bae 100644
--- a/semantics.rkt
+++ b/semantics.rkt
@@ -136,10 +136,13 @@
       (let ([next-st (update-stats incr-stats (step st))])
         (cons st (eval-state next-st)))))
 
+(: data-node? (-> Node Boolean))
+(define data-node? integer?)
+
 (: final-state? (-> State Boolean))
 (define (final-state? st)
   (match (State-stack st)
-    [(list n) (integer-node? (lookup-node (State-heap st) n))]
+    [(list n) (data-node? (lookup-node (State-heap st) n))]
     [_ #f]))
 
 (: step (-> State State))