diff --git a/networks.rkt b/networks.rkt
index c786282..7310750 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -917,3 +917,11 @@
 ;;; where k = n - 1.
 (define (random-boolean-network/vars n)
   (random-boolean-network (for/list ([i (in-range n)]) (string->symbol (format "x~a" i)))))
+
+
+;;; ===================
+;;; TBF/TBN and SBF/SBN
+;;; ===================
+
+;;; A TBN is a network form mapping TBFs to variables.
+(define tbn? (hash/c variable? tbf?))