diff --git a/scribblings/tbn.scrbl b/scribblings/tbn.scrbl
index 6d96ac1..4474b59 100644
--- a/scribblings/tbn.scrbl
+++ b/scribblings/tbn.scrbl
@@ -286,6 +286,21 @@ a @racket[TBF/State].
 
 }
 
+@defproc[(sbn? [tbn TBN]) Boolean]{
+
+A SBN is a @racket[TBN] in which all @racket[TBF/State]s satisfy
+@racket[sbf/state?].
+
+All functions in @racket[tbn] must only reference variables appearing
+in the network.  This function does not check this condition.
+
+@ex[
+(let ([f1 (tbf/state (hash 'a -1 'b 1) 0)]
+      [f2 (tbf/state (hash 'a -1 'b 1) 1)])
+  (values (sbn? (hash 'a f1 'b f1))
+          (sbn? (hash 'a f1 'b f2))))
+]}
+
 @section{Miscellaneous utilities}
 
 @defproc[(group-truth-table-by-nai [tt (Listof (Listof Integer))])
diff --git a/tbn.rkt b/tbn.rkt
index 0ba0288..ceb4120 100644
--- a/tbn.rkt
+++ b/tbn.rkt
@@ -35,7 +35,7 @@
 
    group-truth-table-by-nai
 
-   TBN
+   TBN sbn?
    )
 
   (: apply-tbf-to-state (-> TBF (State (U Zero One)) (U Zero One)))
@@ -321,6 +321,17 @@
                       ((1 1 1 0))))))
 
   (define-type TBN (HashTable Variable TBF/State))
+
+  (: sbn? (-> TBN Boolean))
+  (define (sbn? tbn) (andmap sbf/state? (hash-values tbn)))
+
+  (module+ test
+    (test-case "sbn?"
+      (define f1 (tbf/state (hash 'a -1 'b 1) 0))
+      (define f2 (tbf/state (hash 'a -1 'b 1) 1))
+      (check-true (sbn? (hash 'a f1 'b f1)))
+      (check-false (sbn? (hash 'a f1 'b f2))))
+    )
   )
 
 (module+ test