diff --git a/functions.rkt b/functions.rkt
index 929d083..d627cc8 100644
--- a/functions.rkt
+++ b/functions.rkt
@@ -39,7 +39,10 @@
   [read-org-tbfs (->* (string?) (#:headers boolean?) (listof tbf?))]
   [tbf-tabulate* (-> (listof tbf?) (listof (listof (or/c 0 1))))]
   [tbf-tabulate (-> tbf? (listof (listof (or/c 0 1))))]
-  [tbf-tabulate*/boolean (-> (listof tbf?) (listof (listof boolean?)))]))
+  [tbf-tabulate*/boolean (-> (listof tbf?) (listof (listof boolean?)))])
+ ;; Predicates
+ (contract-out
+  [sbf? (-> any/c boolean?)]))
 
 (module+ test
   (require rackunit))
@@ -366,3 +369,10 @@
   (test-case "tbf-tabulate*/boolean"
     (check-equal? (tbf-tabulate*/boolean `(,(tbf #(1 2) 1)))
                   '((#f #f #f) (#f #t #t) (#t #f #f) (#t #t #t)))))
+
+;;; A sign Boolean function (SBF) is a TBF whose threshold is 0.
+(define sbf? (and/c tbf? (λ (x) (= 0 (tbf-θ x)))))
+
+(module+ test
+  (check-false (sbf? (tbf #(1 2) 3)))
+  (check-true (sbf? (tbf #(1 2) 0))))