diff --git a/os2.rkt b/os2.rkt
index 89b9c21..7a561e5 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -58,7 +58,10 @@
 	 non-wild?
 
 	 ;; Reexports from racket/match for convenience
-	 (all-from-out racket/match))
+	 (all-from-out racket/match)
+
+	 ;; For debugging
+	 current-ground-transition)
 
 ;; Endpoints are the units of deduplication.
 ;; Flows (in canonical form) are the units of presence.
@@ -551,7 +554,7 @@
 
 (define (ground-vm boot)
   (let loop ((state (make-vm 'ground-vm boot)))
-    (match (run-vm state)
+    (match (let ((t (run-vm state))) (set! current-ground-transition t) t)
       [(transition state actions)
        (define is-blocking?
 	 (match actions
@@ -578,3 +581,5 @@
 					(wrap-evt always-evt (lambda (dummy) values)))
 				    active-events)))
 	     (loop (interruptk state))))])))
+
+(define current-ground-transition #f)
\ No newline at end of file