diff --git a/rmacs/diff.rkt b/rmacs/diff.rkt
index e3ea203..e06b9f3 100644
--- a/rmacs/diff.rkt
+++ b/rmacs/diff.rkt
@@ -5,6 +5,8 @@
 ;; comparison, Bell Telephone Laboratories CSTR #41 (1976)
 ;; http://www.cs.dartmouth.edu/~doug/
 
+(provide diff-indices)
+
 (require racket/set)
 (require racket/match)