diff --git a/networks.rkt b/networks.rkt
index fde16b8..0fc6f5a 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -100,7 +100,8 @@
                [update-function-form? (-> any/c boolean?)]
                [network-form? (-> any/c boolean?)]
                [modality? (-> any/c boolean?)]
-               [mode? (-> any/c boolean?)])
+               [mode? (-> any/c boolean?)]
+               [sbf/state? (-> any/c boolean?)])
  ;; Contracts
  (contract-out [state/c contract?]
                [update-function/c contract?]
@@ -959,6 +960,13 @@
 ;;; applied to states in an unambiguous ways.
 (struct tbf/state (weights threshold) #:transparent)
 
+;;; A sign Boolean function (SBF) is a TBF whose threshold is 0.
+(define sbf/state? (and/c tbf/state? (λ (tbf) (zero? (tbf/state-θ tbf)))))
+
+(module+ test
+  (test-case "sbf/state?"
+    (check-true (sbf/state? (tbf/state #hash((a . -1) (b . 1)) 0)))))
+
 ;;; Shortcuts for acessing fields of a state/tbf.
 (define tbf/state-w tbf/state-weights)
 (define tbf/state-θ tbf/state-threshold)