diff --git a/os2-example.rkt b/os2-example.rkt
index 746af01..b4d0e26 100644
--- a/os2-example.rkt
+++ b/os2-example.rkt
@@ -13,7 +13,7 @@
       #:name id
       #:state state
       [(timer-expired (== id) now)
-       (sequence-actions state
+       (sequence-actions (transition state)
 	 k
 	 (delete-role id))])))
 
@@ -32,7 +32,7 @@
 (define spy
   (lambda (spy-pid)
     (define (hs label)
-      (define ((w kind) . args) (write `(,label ,kind ,@args)) (newline) values)
+      (define ((w kind) . args) (write `(,label ,kind ,@args)) (newline) transition)
       (handlers (w 'presence)
 		(w 'absence)
 		(w 'message)))
diff --git a/os2-nested-example.rkt b/os2-nested-example.rkt
index e20b0e9..23ada89 100644
--- a/os2-nested-example.rkt
+++ b/os2-nested-example.rkt
@@ -7,7 +7,7 @@
 (define (spy spy-label)
   (lambda (spy-pid)
     (define (hs label)
-      (define ((w kind) . args) (write `(,spy-label ,label ,kind ,@args)) (newline) values)
+      (define ((w kind) . args) (write `(,spy-label ,label ,kind ,@args)) (newline) transition)
       (handlers (w 'presence)
 		(w 'absence)
 		(w 'message)))
diff --git a/os2-tcp-test-bytewise-echo.rkt b/os2-tcp-test-bytewise-echo.rkt
index 6d34c5b..cbbd195 100644
--- a/os2-tcp-test-bytewise-echo.rkt
+++ b/os2-tcp-test-bytewise-echo.rkt
@@ -11,14 +11,11 @@
 (define ((connection-handler local-addr remote-addr) self-pid)
   (define (reader-role)
     (role (topic-subscriber (tcp-channel remote-addr local-addr (wild)))
-      #:state state
       [(tcp-channel remote _ (? bytes? bs))
-       (transition state
-	 (send-tcp-credit remote-addr local-addr 16)
-	 (send-message (tcp-channel local-addr remote-addr bs)))]
+       (list (send-tcp-credit remote-addr local-addr 16)
+	     (send-message (tcp-channel local-addr remote-addr bs)))]
       [(tcp-channel remote _ (? eof-object?))
-       (transition state
-	 (kill))]))
+       (kill)]))
 
   (transition 'no-state
     (send-tcp-credit remote-addr local-addr 16)
@@ -27,20 +24,18 @@
 (define (listener local-addr)
   (transition 'no-state
     (role (topic-subscriber (tcp-channel (wild) local-addr (wild)) #:monitor? #t)
-      #:state state
       #:topic t
       #:on-presence (match t
 		      [(topic 'publisher (tcp-channel remote-addr (== local-addr) _) #f)
-		       (transition state (spawn (connection-handler local-addr remote-addr)))]))))
+		       (spawn (connection-handler local-addr remote-addr))]))))
 
 (define (main port)
   (define (arm-timer)
     (role (topic-subscriber (timer-expired (wild) (wild)))
       #:name 'waiter
-      #:state state
-      #:on-presence (transition state (send-message (set-timer 'label 500 'relative)))
-      [(timer-expired _ _)
-       (transition state (delete-role 'waiter) (arm-timer))]))
+      #:on-presence (send-message (set-timer 'label 500 'relative))
+      [(timer-expired _ _) (list (delete-role 'waiter)
+				 (arm-timer))]))
   (ground-vm
    (transition 'none
      (spawn tcp-spy)
diff --git a/os2-tcp-test-chat-service.rkt b/os2-tcp-test-chat-service.rkt
index db785da..859130b 100644
--- a/os2-tcp-test-chat-service.rkt
+++ b/os2-tcp-test-chat-service.rkt
@@ -23,13 +23,14 @@
 
 (define (connection-handler local-addr)
   (transition (set) ;; of remote TcpAddresses
+    (role (topic-publisher (tcp-channel local-addr (wild) (wild))))
     (role (topic-subscriber (tcp-channel (wild) local-addr (wild)))
       #:state active-remotes
       #:topic t
       #:on-presence (match t
 		      [(topic 'publisher (tcp-channel (== local-addr) _ _) #f)
 		       ;; Ignore loopback flow.
-		       active-remotes]
+		       (transition active-remotes)]
 		      [(topic 'publisher (tcp-channel remote-addr (== local-addr) _) #f)
 		       (write `(arrived ,remote-addr)) (newline)
 		       (transition (set-add active-remotes remote-addr)
@@ -40,11 +41,11 @@
 			 (send-tcp-credit remote-addr local-addr 1))])
       #:on-absence (match t
 		     [(topic 'publisher (tcp-channel (== local-addr) _ _) #f)
-		       ;; Ignore loopback flow.
-		       active-remotes]
+		      ;; Ignore loopback flow.
+		      (transition active-remotes)]
 		     [(topic 'publisher (tcp-channel remote-addr (== local-addr) _) #f)
 		      (write `(departed ,remote-addr)) (newline)
-		      (set-remove active-remotes remote-addr)])
+		      (transition (set-remove active-remotes remote-addr))])
       [(tcp-channel remote-addr (== local-addr) (? eof-object?))
        (define new-active-remotes (set-remove active-remotes remote-addr))
        (transition new-active-remotes
@@ -53,9 +54,7 @@
       [(tcp-channel remote-addr (== local-addr) (? bytes? bs))
        (transition active-remotes
 	 (send-tcp-credit remote-addr local-addr 1)
-	 (send-to-all local-addr active-remotes remote-addr bs))])
-    (role (topic-publisher (tcp-channel local-addr (wild) (wild)))
-      #:state active-remotes)))
+	 (send-to-all local-addr active-remotes remote-addr bs))])))
 
 (define (main port)
   (ground-vm
diff --git a/os2-tcp-test-date-service.rkt b/os2-tcp-test-date-service.rkt
index 36e8641..1e7311e 100644
--- a/os2-tcp-test-date-service.rkt
+++ b/os2-tcp-test-date-service.rkt
@@ -10,10 +10,7 @@
 (define ((connection-handler local-addr remote-addr) self-pid)
   (transition 'no-state
     (role (topic-publisher (tcp-channel local-addr remote-addr (wild)))
-      #:state state
-      [(tcp-channel _ _ (tcp-credit _))
-       (transition state
-	 (kill))])
+      [(tcp-channel _ _ (tcp-credit _)) (kill)])
     (send-message (tcp-channel local-addr remote-addr
 			       (string->bytes/utf-8
 				(format "~a\n" (current-inexact-milliseconds)))))))
@@ -21,11 +18,10 @@
 (define (listener local-addr)
   (transition 'no-state
     (role (topic-subscriber (tcp-channel (wild) local-addr (wild)) #:monitor? #t)
-      #:state state
       #:topic t
       #:on-presence (match t
 		      [(topic 'publisher (tcp-channel remote-addr (== local-addr) _) #f)
-		       (transition state (spawn (connection-handler local-addr remote-addr)))]))))
+		       (spawn (connection-handler local-addr remote-addr))]))))
 
 (define (main port)
   (ground-vm
diff --git a/os2-tcp-test-echo-service.rkt b/os2-tcp-test-echo-service.rkt
index 75956a4..84917d3 100644
--- a/os2-tcp-test-echo-service.rkt
+++ b/os2-tcp-test-echo-service.rkt
@@ -12,24 +12,20 @@
     (send-tcp-mode remote-addr local-addr 'lines)
     (send-tcp-credit remote-addr local-addr 1)
     (role (topic-subscriber (tcp-channel remote-addr local-addr (wild)))
-      #:state state
       [(tcp-channel remote _ (? bytes? line))
-       (transition state
-	 (send-tcp-credit remote-addr local-addr 1)
-	 (send-message
-	  (tcp-channel local-addr remote-addr (bytes-append #"You said: " line #"\n"))))]
+       (list (send-tcp-credit remote-addr local-addr 1)
+	     (send-message
+	      (tcp-channel local-addr remote-addr (bytes-append #"You said: " line #"\n"))))]
       [(tcp-channel remote _ (? eof-object?))
-       (transition state
-	 (kill))])))
+       (kill)])))
 
 (define (listener local-addr)
   (transition 'no-state
     (role (topic-subscriber (tcp-channel (wild) local-addr (wild)) #:monitor? #t)
-      #:state state
       #:topic t
       #:on-presence (match t
 		      [(topic 'publisher (tcp-channel remote-addr (== local-addr) _) #f)
-		       (transition state (spawn (connection-handler local-addr remote-addr)))]))))
+		       (spawn (connection-handler local-addr remote-addr))]))))
 
 (define (main port)
   (ground-vm
diff --git a/os2-tcp.rkt b/os2-tcp.rkt
index 8e90ed9..5fd8779 100644
--- a/os2-tcp.rkt
+++ b/os2-tcp.rkt
@@ -104,9 +104,9 @@
     [(or (topic 'publisher (tcp-channel local-addr remote-addr _) _)
 	 (topic 'subscriber (tcp-channel remote-addr local-addr _) _))
      (cond
-      [(ground? remote-addr) active-handles]
-      [(not (ground? local-addr)) active-handles]
-      [(set-member? active-handles (cons local-addr remote-addr)) active-handles]
+      [(ground? remote-addr) (transition active-handles)]
+      [(not (ground? local-addr)) (transition active-handles)]
+      [(set-member? active-handles (cons local-addr remote-addr)) (transition active-handles)]
       [else
        (transition (set-add active-handles (cons local-addr remote-addr))
 	 (spawn (driver-fun local-addr remote-addr)
@@ -118,9 +118,9 @@
     [(or (topic 'publisher (tcp-channel local-addr remote-addr _) _)
 	 (topic 'subscriber (tcp-channel remote-addr local-addr _) _))
      (cond
-      [(ground? remote-addr) active-handles]
-      [(not (ground? local-addr)) active-handles]
-      [else (set-remove active-handles local-addr)])]))
+      [(ground? remote-addr) (transition active-handles)]
+      [(not (ground? local-addr)) (transition active-handles)]
+      [else (transition (set-remove active-handles local-addr))])]))
 
 ;; TcpAddress TcpAddress -> BootK
 (define ((tcp-listener-manager local-addr dummy-remote-addr) self-pid)
@@ -140,7 +140,7 @@
 		     [(or (topic 'publisher (tcp-channel (== local-addr) remote-addr _) _)
 			  (topic 'subscriber (tcp-channel remote-addr (== local-addr) _) _))
 		      (if (ground? remote-addr)
-			  state
+			  (transition state)
 			  (transition 'listener-is-closed
 			    (kill)
 			    (when (eq? state 'listener-is-running)
@@ -231,10 +231,10 @@
       [(tcp-channel (== remote-addr) (== local-addr) subpacket)
        (match subpacket
 	 [(tcp-credit amount)
-	  (and state (adjust-credit state amount))]
+	  (if state (adjust-credit state amount) (transition state))]
 	 [(tcp-mode new-mode)
 	  ;; Also resets credit to zero.
-	  (and state (adjust-credit (tcp-connection-state new-mode 0) 0))]
+	  (if state (adjust-credit (tcp-connection-state new-mode 0) 0) (transition state))]
 	 [_
 	  (error 'tcp-connection-manager*
 		 "Subscriber on a channel may only send channel control messages")])])))
@@ -247,17 +247,16 @@
   (transition 'no-state
     (role (set (topic-subscriber (wild) #:monitor? #t)
 	       (topic-publisher (wild) #:monitor? #t))
-      #:state state
       [(tcp-channel source dest (? bytes? body))
        (write `(TCPDATA ,source --> ,dest)) (newline)
        (dump-bytes! body (bytes-length body))
-       state]
+       (void)]
       [(tcp-channel source dest (? eof-object?))
        (write `(TCPEOF ,source --> ,dest)) (newline)
-       state]
+       (void)]
       [(tcp-channel source dest (tcp-credit amount))
        (write `(TCPCREDIT ,source --> ,dest ,amount)) (newline)
-       state]
+       (void)]
       [other
        (write `(TCPOTHER ,other)) (newline)
-       state])))
+       (void)])))
diff --git a/os2-testing.rkt b/os2-testing.rkt
index 9e79a05..31e6401 100644
--- a/os2-testing.rkt
+++ b/os2-testing.rkt
@@ -9,9 +9,7 @@
 (provide check-role)
 
 (define (flatten-transition t)
-  (if (transition? t)
-      (apply transition (transition-state t) (flatten (transition-actions t)))
-      (transition t)))
+  (apply transition (transition-state t) (flatten (transition-actions t))))
 
 ;; AddRole World SendMessage World ConsTreeOf<Action> -> Void
 ;; Passes the given message to the given message-handler, with the
diff --git a/os2-timer.rkt b/os2-timer.rkt
index 0c92bea..7d392b7 100644
--- a/os2-timer.rkt
+++ b/os2-timer.rkt
@@ -110,7 +110,7 @@
 	   [(cons (? evt?) now)
 	    (define to-send (fire-timers! (driver-state-heap state) now))
 	    ;; Note: compute to-send before recursing, because of side-effects on heap
-	    (sequence-actions state
+	    (sequence-actions (transition state)
 	      update-time-listener!
 	      to-send)]))))
 
diff --git a/os2-udp-test.rkt b/os2-udp-test.rkt
index 7a323b5..67e42a7 100644
--- a/os2-udp-test.rkt
+++ b/os2-udp-test.rkt
@@ -10,14 +10,11 @@
 (define (packet-handler local-addr)
   (role (set (topic-publisher (udp-packet local-addr (wild) (wild)))
 	     (topic-subscriber (udp-packet (wild) local-addr (wild))))
-    #:state state
     [(udp-packet source _ #"quit\n")
-     (transition state
-       (send-message (udp-packet local-addr source #"OK, quitting\n"))
-       (kill #:reason "Asked to quit"))]
+     (list (send-message (udp-packet local-addr source #"OK, quitting\n"))
+	   (kill #:reason "Asked to quit"))]
     [(udp-packet source sink body)
-     (transition state
-       (send-message (udp-packet sink source body)))]))
+     (send-message (udp-packet sink source body))]))
 
 (check-role (packet-handler (udp-listener 5555))
 	    'arbitrary
diff --git a/os2-udp.rkt b/os2-udp.rkt
index 058061a..7f24615 100644
--- a/os2-udp.rkt
+++ b/os2-udp.rkt
@@ -58,7 +58,7 @@
       #:on-presence (match t
 		      [(topic _ (udp-packet _ local-addr _) #f)
 		       (cond
-			[(set-member? active-handles local-addr) active-handles]
+			[(set-member? active-handles local-addr) (transition active-handles)]
 			[else
 			 (transition (set-add active-handles local-addr)
 			   (spawn (udp-socket-manager local-addr)
@@ -102,7 +102,7 @@
 			      #:debug-name (list 'udp-socket-closer local-addr))))
       [(udp-packet (== local-addr) (udp-address remote-host remote-port) body)
        (udp-send-to s remote-host remote-port body)
-       state])
+       (transition state)])
     ;; Listen for messages arriving on the actual socket using a
     ;; ground event, and relay them at this level.
     (role (topic-subscriber (cons (udp-receive!-evt s buffer) (wild)))
@@ -119,11 +119,10 @@
 (define udp-spy
   (transition 'no-state
     (role (topic-subscriber (wild) #:monitor? #t)
-      #:state state
       [(udp-packet source dest body)
        (write `(UDP ,source --> ,dest)) (newline)
        (dump-bytes! body (bytes-length body))
-       state]
+       (void)]
       [other
        (write `(UDP ,other)) (newline)
-       state])))
+       (void)])))
diff --git a/os2.rkt b/os2.rkt
index 53988f2..ceb9ec1 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -174,7 +174,9 @@
 ;; Transition -> (transition ConsTreeOf<Action>)
 (define (maybe-transition->transition t)
   (cond [(transition? t) t]
-	[else (transition t '())]))
+	[else
+	 (define message (format "maybe-transition->transition: Expected transition; got ~v" t))
+	 (transition #f (kill #f (exn:fail:contract message (current-continuation-marks))))]))
 
 ;; Preactions.
 ;; Ks are various TrapKs or #f, signifying lack of interest.
@@ -277,7 +279,7 @@
 				   (match message-body
 				     [message-pattern clause-body ...]
 				     ...
-				     [_ state])]))))])
+				     [_ (make-transition state)])]))))])
 	 #`(add-role #,(if (attribute pre-eid)
 			   #'pre-eid
 			   #'(gensym 'anonymous-role))
@@ -407,7 +409,7 @@
 ;; Composing state transitions and action emissions.
 
 (define (sequence-actions t . more-actions-and-transformers)
-  (match-define (transition initial-state initial-actions) (maybe-transition->transition t))
+  (match-define (transition initial-state initial-actions) t)
   (let loop ((state initial-state)
 	     (actions initial-actions)
 	     (items more-actions-and-transformers))
@@ -415,8 +417,7 @@
       ['()
        (transition state actions)]
       [(cons (? procedure? transformer) remaining-items)
-       (match-define (transition new-state more-actions)
-	 (maybe-transition->transition (transformer state)))
+       (match-define (transition new-state more-actions) (transformer state))
        (loop new-state
 	     (cons actions more-actions)
 	     remaining-items)]
@@ -724,7 +725,7 @@
 (define (((wrap-trapk pid new-party trapk) . args) state)
   (if (hash-has-key? (vm-processes state) pid)
       (run-vm (apply run-trapk state pid new-party trapk args))
-      state))
+      (make-transition state)))
 
 (define (transform-meta-action pid preaction state)
   (match preaction