diff --git a/os2.rkt b/os2.rkt
index 356ec46..cb3cf17 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -246,6 +246,8 @@
 (define (topic-subscriber pattern #:virtual? [virtual? #f])
   (topic 'subscriber pattern virtual?))
 
+;; TODO: Ideally this would be extensible; roles like 'debug-listener,
+;; 'logger etc exist.
 (define (co-roles r)
   (case r
     [(publisher) '(subscriber)]