diff --git a/planner/src/step.sld b/planner/src/step.sld
index f4db2f5..a7e3608 100644
--- a/planner/src/step.sld
+++ b/planner/src/step.sld
@@ -8,6 +8,8 @@
   register-build-step
 
   empty-build-plan build-plan-print build-plan-tick
+  build-plan-next-id
+
   build-plan-current-plan
 
   build-plan-append build-plan-tail-call build-plan-return-append)
@@ -40,10 +42,15 @@
   (define (empty-build-plan)
     (make-build-plan (mapping (make-default-comparator)) '() (mapping (make-default-comparator)) 0))
   
-  ;; Appends a new step to the build plan, returning its ID.
-  (define (build-plan-append plan type args dependencies)
+  ;; Returns a monotonically-increasing identifier for this build plan.
+  (define (build-plan-next-id plan)
     (define id (+ (build-plan-last-id plan) 1))
     (set-build-plan-last-id! plan id)
+    id)
+
+  ;; Appends a new step to the build plan, returning its ID.
+  (define (build-plan-append plan type args dependencies)
+    (define id (build-plan-next-id plan))
     (define new-step (make-build-step id dependencies type args))
     (set-build-plan-steps! plan (mapping-set! (build-plan-steps plan) id new-step))
     id)