diff --git a/os2.rkt b/os2.rkt
index e2540ec..4fa1293 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -242,8 +242,8 @@
   (lambda (stx)
     (syntax-parse stx
       [(_ pre-eid topics-expr
-	  #:state state-pattern
-	  (~or (~optional (~seq #:on-presence presence) #:name "#:on-presence handler")
+	  (~or (~optional (~seq #:state state-pattern) #:name "#:state pattern")
+	       (~optional (~seq #:on-presence presence) #:name "#:on-presence handler")
 	       (~optional (~seq #:on-absence absence) #:name "#:on-absence handler")
 	       (~optional (~seq #:topic topic) #:defaults ([topic #'t0]) #:name "#:topic")
 	       (~optional (~seq #:reason reason) #:defaults ([reason #'r0]) #:name "#:reason"))
@@ -251,19 +251,32 @@
 	  [message-pattern clause-body ...]
 	  ...)
        (define-syntax-rule (build-handler args e-attr)
-	 (if (not (attribute e-attr))
-	     #'#f
-	     #`(lambda args (match-lambda [state-pattern e-attr]))))
+	 (cond
+	  [(not (attribute e-attr))
+	   #'#f]
+	  [(not (attribute state-pattern))
+	   #`(lambda args (match-lambda [state (transition state e-attr)]))]
+	  [else
+	   #`(lambda args (match-lambda [state-pattern e-attr]))]))
        (with-syntax ([presence-handler (build-handler (topic) presence)]
 		     [absence-handler (build-handler (topic reason) absence)]
-		     [message-handler #'(lambda (topic message-body)
-					  (lambda (state)
-					    (match state
-					      [state-pattern
-					       (match message-body
-						 [message-pattern clause-body ...]
-						 ...
-						 [_ state])])))])
+		     [message-handler
+		      (if (not (attribute state-pattern))
+			  #'(lambda (topic message-body)
+			      (lambda (state)
+				(transition state
+				  (match message-body
+				    [message-pattern clause-body ...]
+				    ...
+				    [_ '()]))))
+			  #'(lambda (topic message-body)
+			      (lambda (state)
+				(match state
+				  [state-pattern
+				   (match message-body
+				     [message-pattern clause-body ...]
+				     ...
+				     [_ state])]))))])
 	 #'(add-role pre-eid
 		     topics-expr
 		     (handlers presence-handler absence-handler message-handler)))])))