diff --git a/os.rkt b/os.rkt
index e265757..fae4e2d 100644
--- a/os.rkt
+++ b/os.rkt
@@ -23,6 +23,7 @@
  (struct-out ground-event-pattern)
  (struct-out ground-event-value)
  ground-vm
+ current-ground-transition ;; DEBUG
  )
 
 ;; Each VM hosts 0 or more *multiplexed* processes. Each process has
@@ -360,6 +361,7 @@
   (let loop ((transition (run-vm (make-vm boot
 					  #:pattern-predicate pattern-predicate
 					  #:meta-pattern-predicate match-ground-event))))
+    (set! current-ground-transition transition)
     (for-each (lambda (thunk) (thunk)) (kernel-mode-transition-messages transition))
     (when (not (nested-vm-inert? (kernel-mode-transition-suspension transition)))
       (match transition
@@ -384,3 +386,6 @@
 	[_
 	 (error 'ground-vm
 		"Outermost VM may not spawn new siblings or send or receive metamessages")]))))
+
+;; For debugging
+(define current-ground-transition #f)