diff --git a/os2.rkt b/os2.rkt
index ae74429..2c7efbd 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -14,6 +14,7 @@
 	 topic-subscriber
 	 co-roles
 	 co-topics
+	 topic-union
 
 	 (struct-out handlers)
 
@@ -53,7 +54,10 @@
 ;; process's PID and the second being an integer. (Except for the
 ;; ground-vm, where they're different because there aren't any PIDs.)
 
-;; One endpoint, one topic.
+;; One endpoint, one topic, with the caveat that as we are at present
+;; unable to represent true topic unions, we actually store a *set* of
+;; topics against each endpoint. The topic for the endpoint is to be
+;; taken as the union of all the members in the set.
 
 ;; A Flow is a Topic that comes from the intersection of two dual
 ;; topics.
@@ -66,10 +70,10 @@
 	    pending-actions ;; QuasiQueue<(cons PID Action)>
 	    ) #:transparent)
 
-;; An Endpoint is an (endpoint EID Topic Handlers), representing a
+;; An Endpoint is an (endpoint EID Set<Topic> Handlers), representing a
 ;; facet of a process responsible for playing a particular role (the
 ;; topic) in a conversation.
-(struct endpoint (id topic handlers) #:transparent)
+(struct endpoint (id topics handlers) #:transparent)
 
 ;; A Process is an Exists State . (process PID State
 ;; NonnegativeInteger Set<EID>), representing a VM process and its
@@ -94,7 +98,7 @@
 
 ;; Preactions.
 ;; Ks are various TrapKs or #f, signifying lack of interest.
-(struct add-role (topic handlers k) #:prefab)
+(struct add-role (topics handlers k) #:prefab)
 (struct delete-role (eid reason) #:prefab)
 (struct send-message (topic body) #:prefab)
 (struct spawn (thunk k) #:prefab)
@@ -111,7 +115,7 @@
 (define-syntax role
   (lambda (stx)
     (syntax-parse stx
-      [(_ topic-expr
+      [(_ topics-expr
 	  #:state state-pattern
 	  (~or (~optional (~seq #:on-presence presence) #:name "#:on-presence handler")
 	       (~optional (~seq #:on-absence absence) #:name "#:on-absence handler")
@@ -137,7 +141,7 @@
 						 [message-pattern clause-body ...]
 						 ...
 						 [_ state])])))])
-	 #'(add-role topic-expr
+	 #'(add-role topics-expr
 		     (handlers presence-handler absence-handler message-handler)
 		     ready-handler))])))
 
@@ -145,7 +149,7 @@
 ;; Smarter constructors for transitions and preactions.
 
 (define (make-transition state . actions) (transition state actions))
-(define (make-add-role topic handlers [k #f]) (add-role topic handlers k))
+(define (make-add-role topics handlers [k #f]) (add-role topics handlers k))
 (define (make-delete-role eid [reason #f]) (delete-role eid reason))
 (define (make-spawn thunk [k #f]) (spawn thunk k))
 (define (make-kill [pid #f] [reason #f]) (kill pid reason))
@@ -174,6 +178,11 @@
   (for/list ([co-role (co-roles (topic-role t))])
     (struct-copy topic t [role co-role])))
 
+(define (topic-union . ts)
+  (unless (andmap topic? ts)
+    (error 'topic-union "Expects topics as arguments, but given ~v" ts))
+  (list->set ts))
+
 (define (refine-topic remote-topic new-pattern)
   (struct-copy topic remote-topic [pattern new-pattern]))
 
@@ -228,15 +237,23 @@
   (with-handlers ([exn:fail? failure-proc])
     (apply f args)))
 
+(define (ensure-topic-union t)
+  (cond [(topic? t) (set t)]
+	[(set? t) t]
+	[else
+	 (error 'ensure-topic-union
+		"Expected either a single topic or a set of topics; got ~v"
+		t)]))
+
 (define (perform-action pid preaction state)
   (match preaction
-    [(add-role topic hs k) (do-subscribe pid topic hs k state)]
+    [(add-role topics hs k) (do-subscribe pid (ensure-topic-union topics) hs k state)]
     [(delete-role eid reason) (do-unsubscribe pid eid reason state)]
     [(send-message topic body) (route-and-deliver topic body state)]
     [(spawn thunk k) (do-spawn pid thunk k state)]
     [(kill pid-to-kill reason) (do-kill (or pid-to-kill pid) reason state)]))
 
-(define (do-subscribe pid topic hs k state)
+(define (do-subscribe pid topics hs k state)
   (cond
    [(hash-has-key? (vm-processes state) pid)
     (define old-process (hash-ref (vm-processes state) pid))
@@ -246,7 +263,9 @@
 			([(matching-pid p) (in-hash (vm-processes state))]
 			 [matching-eid (in-set (process-endpoints p))]
 			 [e (in-value (hash-ref (vm-endpoints state) matching-eid))]
-			 [matching-topic (in-value (endpoint-topic e))]
+			 [matching-topics (in-value (endpoint-topics e))]
+			 [matching-topic (in-set matching-topics)]
+			 [topic (in-set topics)]
 			 [flow-pattern (in-value (topic-intersection topic matching-topic))]
 			 #:when flow-pattern)
 		      (define inbound-flow (refine-topic matching-topic flow-pattern))
@@ -276,7 +295,7 @@
 		 [endpoints (hash-set (vm-endpoints state)
 				      new-eid
 				      (endpoint new-eid
-						topic
+						topics
 						hs))])]
    [else state]))
 
@@ -284,7 +303,7 @@
   (cond
    [(hash-has-key? (vm-endpoints state) eid)
     (define endpoint-to-remove (hash-ref (vm-endpoints state) eid))
-    (define removed-topic (endpoint-topic endpoint-to-remove))
+    (define removed-topics (endpoint-topics endpoint-to-remove))
     (define old-process (hash-ref (vm-processes state) pid))
     (define new-process (struct-copy process old-process
 				     [endpoints (set-remove (process-endpoints old-process) eid)]))
@@ -295,7 +314,9 @@
 	  ([(matching-pid p) (in-hash (vm-processes state))]
 	   [matching-eid (in-set (process-endpoints p))]
 	   [e (in-value (hash-ref (vm-endpoints state) matching-eid))]
-	   [matching-topic (in-value (endpoint-topic e))]
+	   [matching-topics (in-value (endpoint-topics e))]
+	   [matching-topic (in-set matching-topics)]
+	   [removed-topic (in-set removed-topics)]
 	   [flow-pattern (in-value (topic-intersection removed-topic matching-topic))]
 	   #:when flow-pattern)
 	(define outbound-flow (refine-topic removed-topic flow-pattern))
@@ -312,7 +333,8 @@
     (for*/set ([(matching-pid p) (in-hash (vm-processes state))]
 	       [matching-eid (in-set (process-endpoints p))]
 	       [e (in-value (hash-ref (vm-endpoints state) matching-eid))]
-	       [matching-topic (in-value (endpoint-topic e))]
+	       [matching-topics (in-value (endpoint-topics e))]
+	       [matching-topic (in-set matching-topics)]
 	       [flow-pattern (in-value (topic-intersection message-topic matching-topic))]
 	       #:when flow-pattern)
       (cons matching-pid e)))
@@ -384,8 +406,8 @@
 
 (define (transform-meta-action pid preaction)
   (match preaction
-    [(add-role topic hs k)
-     (add-role topic
+    [(add-role topics hs k)
+     (add-role topics
 	       (handlers (wrap-trapk pid (handlers-presence hs))
 			 (wrap-trapk pid (handlers-absence hs))
 			 (wrap-trapk pid (handlers-message hs)))
@@ -406,11 +428,12 @@
        (when (not (null? actions))
 	 (error 'ground-vm "No meta-actions available in ground-vm: ~v" actions))
        (define waiting? (null? (vm-pending-actions state)))
-       (define active-events (for/list ([(eid e) (in-hash (vm-endpoints state))]
-					#:when (and (evt? (topic-pattern (endpoint-topic e)))
-						    (eq? (topic-role (endpoint-topic e))
-							 'subscriber)))
-			       (define evt (topic-pattern (endpoint-topic e)))
+       (define active-events (for*/list ([(eid e) (in-hash (vm-endpoints state))]
+					 [topic (in-set (endpoint-topics e))]
+					 #:when (and (evt? (topic-pattern topic))
+						     (eq? (topic-role topic)
+							  'subscriber)))
+			       (define evt (topic-pattern topic))
 			       (wrap-evt evt (lambda (message)
 					       (lambda (state)
 						 (route-and-deliver (topic-publisher evt)