diff --git a/bn.rkt b/bn.rkt
index 5e523e3..d318804 100644
--- a/bn.rkt
+++ b/bn.rkt
@@ -10,6 +10,11 @@
 (provide Variable State UpdateFunc Network
          update make-state make-bn)
 
+
+;;; =================
+;;; Basic definitions
+;;; =================
+
 (define-type Variable Symbol)
 
 ;;; A state of a Boolean network is a mapping from the variables of the
@@ -45,3 +50,18 @@
 (: make-bn (-> (Listof (Pairof Variable UpdateFunc)) Network))
 (define (make-bn funcs)
   (make-hash funcs))
+
+
+;;; =========================================
+;;; Syntactic description of Boolean networks
+;;; =========================================
+
+;;; An UpdateFuncForm is any form which can appear as a body of a
+;;; Boolean function and which can be evaluated with eval.  For
+;;; example, '(and x y (not z)).
+(define-type UpdateFuncForm Any)
+
+;;; A Boolean network form is a mapping from its variables to the
+;;; forms of their update functions.
+(define-type NetworkForm (HashTable Variable UpdateFuncForm))
+