diff --git a/utils.rkt b/utils.rkt
index 5d8a99d..94f03f7 100644
--- a/utils.rkt
+++ b/utils.rkt
@@ -19,6 +19,7 @@
 ;;; usage of hash tables mapping symbols to values.  The goal is
 ;;; essentially to avoid having to write explicit hash-ref calls.
 
+;;; A variable mapping is a hash table mapping symbols to values.
 (define (variable-mapping? dict)
   (and (hash? dict)
        (if (not (hash-empty? dict))