diff --git a/networks.rkt b/networks.rkt
index 32f8a30..93db941 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -14,12 +14,10 @@
          graph racket/random racket/hash)
 
 (provide
- (struct-out dynamics)
  ;; Structures
+ (struct-out dynamics)
  (contract-out [struct tbf/state ([weights (hash/c variable? number?)]
-                                  [threshold number?])]
-               [struct network ([functions (hash/c variable? procedure?)]
-                                [domains domain-mapping/c])])
+                                  [threshold number?])])
  ;; Functions
  (contract-out [update (-> network? state? (set/c variable? #:kind 'dont-care) state?)]
                [make-state (-> (listof (cons/c symbol? any/c)) state?)]
@@ -168,9 +166,8 @@
 ;;; state.
 (define update-function/c (-> state? any/c))
 
-;;; A network is a mapping from its variables to its update functions,
-;;; together with the domains of the variables.
-(struct network (functions domains))
+;;; A network is a mapping from its variables to its update functions.
+(define network? (hash/c variable? procedure?))
 
 ;;; Given a state s updates all the variables from xs.  This
 ;;; corresponds to a parallel mode.