diff --git a/os2.rkt b/os2.rkt
index 4e3d3b1..b295e75 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -3,6 +3,7 @@
 
 (require racket/set)
 (require racket/match)
+(require (only-in racket/list flatten))
 (require "unify.rkt")
 
 ;; Endpoints are the units of deduplication.
@@ -53,7 +54,8 @@
 ;; MessageHandler = TrapK<EID * Topic * Message>
 (struct handlers (presence absence message) #:transparent)
 
-;; actions is a plain old List<Action>, not a QuasiQueue.
+;; actions is a plain old ordered ConsTreeOf<Action>, not a
+;; QuasiQueue.
 (struct transition (state actions) #:transparent)
 
 ;; Preactions.
@@ -260,7 +262,7 @@
 
 (define (enqueue-actions state pid actions)
   (struct-copy vm state
-	       [pending-actions (quasi-enqueue-many (for/list ([a actions]) (cons pid a))
+	       [pending-actions (quasi-enqueue-many (for/list ([a (flatten actions)]) (cons pid a))
 						    (vm-pending-actions state))]))
 
 (define (wrap-trapk pid trapk)