diff --git a/functions.rkt b/functions.rkt
index 3a1b5d2..44d9872 100644
--- a/functions.rkt
+++ b/functions.rkt
@@ -31,7 +31,7 @@
 
    (struct-out tbf) tbf-w tbf-θ boolean->01/vector apply-tbf apply-tbf/boolean
    list->tbf lists->tbfs read-org-tbfs tbf-tabulate* tbf-tabulate
-   tbf-tabulate*/boolean)
+   tbf-tabulate*/boolean sbf?)
 
   (module+ test
     (require typed/rackunit))
@@ -539,6 +539,15 @@
       (check-equal? (tbf-tabulate*/boolean (list (tbf #(1 2) 1)))
                     '((#f #f #f) (#f #t #t) (#t #f #f) (#t #t #t)))))
 
+  (: sbf? (-> tbf Boolean))
+  (define (sbf? t)
+    (= 0 (tbf-θ t)))
+
+  (module+ test
+    (test-case "sbf?"
+      (check-false (sbf? (tbf #(1 2) 3)))
+      (check-true (sbf? (tbf #(1 2) 0)))))
+
   (module untyped racket
     (module+ test
       (require rackunit))
@@ -629,7 +638,7 @@
 
  (struct-out tbf) tbf-w tbf-θ boolean->01/vector apply-tbf apply-tbf/boolean
  list->tbf lists->tbfs read-org-tbfs tbf-tabulate* tbf-tabulate
- tbf-tabulate*/boolean)
+ tbf-tabulate*/boolean sbf?)
 
 (require (rename-in (submod 'typed untyped)
                     [tabulate tabulate/untyped]
@@ -640,10 +649,7 @@
  (contract-out
   [sbf (-> (vectorof number?) tbf?)]
   [list->sbf (-> (listof number?) sbf?)]
-  [read-org-sbfs (->* (string?) (#:headers boolean?) (listof sbf?))])
- ;; Predicates
- (contract-out
-  [sbf? (-> any/c boolean?)]))
+  [read-org-sbfs (->* (string?) (#:headers boolean?) (listof sbf?))]))
 
 (module+ test
   (require rackunit))
@@ -653,14 +659,6 @@
 ;;; Threshold Boolean functions
 ;;; ===========================
 
-;;; 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
-  (test-case "sbf?"
-    (check-false (sbf? (tbf #(1 2) 3)))
-    (check-true (sbf? (tbf #(1 2) 0)))))
-
 ;;; Creates a TBF which is an SBF from a vector of weights.
 (define (sbf w) (tbf w 0))
 
diff --git a/scribblings/functions.scrbl b/scribblings/functions.scrbl
index a12d149..2e34d18 100644
--- a/scribblings/functions.scrbl
+++ b/scribblings/functions.scrbl
@@ -565,6 +565,15 @@ TBF in the list.
 (tbf-tabulate*/boolean (list (tbf #(1 2) 1)))
 ]}
 
+@defproc[(sbf? [t tbf]) Boolean]{
+
+A sign Boolean function (SBF) is a TBF whose threshold is 0.
+
+@ex[
+(sbf? (tbf #(1 2) 3))
+(sbf? (tbf #(1 2) 0))
+]}
+
 @section[#:tag "fuctions/untyped"]{Untyped definitions}
 
 @defmodule[(submod dds/functions typed untyped)]