diff --git a/bn.rkt b/bn.rkt
index c759769..d8b3c5a 100644
--- a/bn.rkt
+++ b/bn.rkt
@@ -7,11 +7,13 @@
 ;;; are updated according to their corresponding update functions.
 ;;; The variables to be updated at each step are given by the mode.
 
-(provide State UpdateFunc Network)
+(provide Variable State UpdateFunc Network)
+
+(define-type Variable Symbol)
 
 ;;; A state of a Boolean network is a mapping from the variables of the
 ;;; network to their Boolean values.
-(define-type State (HashTable Symbol Boolean))
+(define-type State (HashTable Variable Boolean))
 
 ;;; An update function is a Boolean function computing a Boolean value
 ;;; from the given state.
@@ -19,4 +21,4 @@
 
 ;;; A Boolean network is a mapping from its variables to its update
 ;;; functions.
-(define-type Network (HashTable Symbol UpdateFunc))
+(define-type Network (HashTable Variable UpdateFunc))