diff --git a/presence/unify.rkt b/presence/unify.rkt
index a0e2abc..ce7578e 100644
--- a/presence/unify.rkt
+++ b/presence/unify.rkt
@@ -164,6 +164,10 @@
   (and s (apply-subst s a-env)))
 
 ;; Utility used by freshen and canonicalize below.
+;; Must visit the term in the order specified by canonicalize
+;; below. Here we rely both upon Racket's left-to-right evaluation
+;; order and upon defined struct-mappers traversing their arguments in
+;; some deterministic order.
 (define (freshen* t var-handler canon-handler)
   (define env (make-hash)) ;; cheeky use of mutation
   (let walk ((t t))