diff --git a/generic.rkt b/generic.rkt
index 44763f7..15bdbe5 100644
--- a/generic.rkt
+++ b/generic.rkt
@@ -19,7 +19,7 @@
 ;;; Given a dds and a set of starting states, produce the set of
 ;;; states reachable in one step.  This is a fallback for dds-step.
 (define (fallback-dds-step dds ss)
-  (list->set (flatten (for/list ([s ss]) (dds-step-one dds s)))))
+  (apply set-union (for/list ([s ss]) (dds-step-one dds s))))
 
 ;;; A discrete dynamical system.
 (define-generics dds