Type sbf/state?.

scribblings/tbn.scrbl

@@ -81,3 +81,13 @@ and weights, as well as a threshold.
 @ex[
 (make-tbf/state '((x1 . 1) (x2 . 1)) 1)
 ]}
+
+@defproc[(sbf/state? [tbfs TBF/State]) Boolean]{
+
+A state sign Boolean function (SBF) is a @racket[TBF/State] whose
+threshold is 0.
+
+@ex[
+(sbf/state? (tbf/state (hash 'a -1 'b 1) 0))
+(sbf/state? (tbf/state (hash 'a -1 'b 1) 1))
+]}

tbn.rkt

@@ -16,6 +16,7 @@
    apply-tbf-to-state
 
    (struct-out tbf/state) TBF/State tbf/state-w tbf/state-θ make-tbf/state
+   sbf/state?
    )
 
   (: apply-tbf-to-state (-> TBF (State (U Zero One)) (U Zero One)))
@@ -45,6 +46,14 @@
       (define f (make-tbf/state '((x1 . 1) (x2 . 1)) 1))
       (check-equal? (tbf/state-w f) #hash((x1 . 1) (x2 . 1)))
       (check-equal? (tbf/state-θ f) 1)))
+
+  (: sbf/state? (-> TBF/State Boolean))
+  (define (sbf/state? tbfs) (zero? (tbf/state-θ tbfs)))
+
+  (module+ test
+    (test-case "sbf/state?"
+      (check-true (sbf/state? (tbf/state (hash 'a -1 'b 1) 0)))
+      (check-false (sbf/state? (tbf/state (hash 'a -1 'b 1) 1)))))
   )
 
 (module+ test