diff --git a/networks.rkt b/networks.rkt
index f625e59..96b80c7 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -1230,5 +1230,8 @@
                     (1 0 0)
                     (1 1 0)))))
 
-;;; A TBN is a network form mapping TBFs to variables.
-(define tbn? (hash/c variable? tbf?))
+;;; A TBN is a network form mapping variables to tbf/state.
+;;;
+;;; The tbf/state must only reference variables appearing in the
+;;; network.  This contract does not check this condition.
+(define tbn? (hash/c variable? tbf/state?))