diff --git a/topicset.rkt b/topicset.rkt
index 5c9ba39..0e3e5c9 100644
--- a/topicset.rkt
+++ b/topicset.rkt
@@ -185,6 +185,10 @@
 (check-equal? (// (list 'x) (list 'y)) (list (// 'x 'y)))
 (check-equal? (// (list 'x) (list 'y)) (list (// 'y 'x)))
 
+(check-false (pair? (// (cons 'x 'y) (cons 'z 'w) (cons 'z 'q))))
+(check-equal? (// (cons 'x 'y) (cons 'z 'w) (cons 'z 'q))
+	      (// (cons 'x 'y) (cons 'z (// 'w 'q))))
+
 (check-equal? (topicset-union (list 'x) (list (// 'x 'y))) (list (// 'x 'y)))
 (check-equal? (topicset-union (list 'y) (list (// 'x 'y))) (list (// 'x 'y)))
 (check-equal? (topicset-union (list 'z) (list (// 'x 'y))) (list (// 'x 'y 'z)))