diff --git a/networks.rkt b/networks.rkt
index 344fc53..447c187 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -20,7 +20,9 @@
                [struct dynamics ([network network?]
                                  [mode mode?])]
                [struct network ([functions (hash/c variable? procedure?)]
-                                [domains domain-mapping/c])])
+                                [domains domain-mapping/c])]
+               [struct network-form ([forms variable-mapping?]
+                                     [domains domain-mapping/c])])
  ;; Functions
  (contract-out [update (-> network? state? (set/c variable? #:kind 'dont-care) state?)]
                [make-state (-> (listof (cons/c symbol? any/c)) state?)]
@@ -236,9 +238,10 @@
 ;;; '(and x y (not z)) or '(+ 1 a (- b 10)).
 (define update-function-form? any/c)
 
-;;; A Boolean network form is a mapping from its variables to the
-;;; forms of their update functions.
-(define network-form? variable-mapping?)
+;;; A network form consists of a mapping from variables to the forms
+;;; of their update functions, together with a mapping from its
+;;; variables to its update functions.
+(struct network-form (forms domains))
 
 ;;; Build an update function from an update function form.
 (define (update-function-form->update-function form)