diff --git a/indenting2.el b/indenting2.el
index f6b60c0..6f42fef 100644
--- a/indenting2.el
+++ b/indenting2.el
@@ -1,6 +1,6 @@
 ;; Emacs indent settings
 (progn
   (mapcar #'(lambda (x) (put x 'scheme-indent-function 1))
-	  '(transition extend-transition))
+	  '(transition extend-transition prefix-transition))
   (mapcar #'(lambda (x) (put x 'scheme-indent-function 2))
 	  '(role role/fresh yield)))
diff --git a/os2.rkt b/os2.rkt
index 7a561e5..0126b9a 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -21,6 +21,7 @@
 	 (except-out (struct-out transition) transition)
 	 (rename-out [make-transition transition])
 	 extend-transition
+	 prefix-transition
 
 	 role
 	 role/fresh
@@ -212,6 +213,11 @@
     [(transition state actions) (transition state (list actions more-actions))]
     [state (transition state more-actions)]))
 
+(define (prefix-transition t . more-actions)
+  (match t
+    [(transition state actions) (transition state (list more-actions actions))]
+    [state (transition state more-actions)]))
+
 ;;---------------------------------------------------------------------------
 ;; Topics and roles