diff --git a/bn-tests.rkt b/bn-tests.rkt
index f7867db..0b80fdd 100644
--- a/bn-tests.rkt
+++ b/bn-tests.rkt
@@ -4,4 +4,30 @@
 
 (require typed/rackunit "bn.rkt")
 
-(check-equal? (+ 1 2 3) 6)
+;;; This test case sets up the following Boolean network:
+;;; x1 = x1 AND NOT x2
+;;; x2 = NOT x2
+(test-begin
+  (test-case "Simple two-variable Boolean network"
+    (let* ([f1 (λ ([s : State])
+                 (let ([x1 (hash-ref s 'x1)]
+                       [x2 (hash-ref s 'x2)])
+                   (and x1 (not x2))))]
+           [f2 (λ ([s : State])
+                 (let ([x2 (hash-ref s 'x2)])
+                   (not x2)))]
+           [bn (hash 'x1 f1 'x2 f2)])
+
+      (test-case "One-step syncronous update"
+        (let* ([s (hash 'x1 #t 'x2 #f)]
+               [new-s (update bn s '(x2 x1))])
+          (check-equal? (hash-ref new-s 'x1) #t)
+          (check-equal? (hash-ref new-s 'x2) #t)
+          (check-equal? (length (hash-keys new-s)) 2)))
+
+      (test-case "One-step asynchronous update"
+        (let* ([s (hash 'x1 #f 'x2 #f)]
+               [new-s (update bn s '(x2 x1))])
+          (check-equal? (hash-ref new-s 'x1) #f)
+          (check-equal? (hash-ref new-s 'x2) #t)
+          (check-equal? (length (hash-keys new-s)) 2))))))
diff --git a/bn.rkt b/bn.rkt
index d8b3c5a..bec7be5 100644
--- a/bn.rkt
+++ b/bn.rkt
@@ -7,7 +7,8 @@
 ;;; are updated according to their corresponding update functions.
 ;;; The variables to be updated at each step are given by the mode.
 
-(provide Variable State UpdateFunc Network)
+(provide Variable State UpdateFunc Network
+         update)
 
 (define-type Variable Symbol)
 
@@ -22,3 +23,14 @@
 ;;; A Boolean network is a mapping from its variables to its update
 ;;; functions.
 (define-type Network (HashTable Variable UpdateFunc))
+
+;;; Given a state s updates all the variables from xs.  This
+;;; corresponds to a parallel mode.
+(define (update [bn : Network]            ; the Boolean network
+                [s : State]               ; the state to operate on
+                [xs : (Listof Variable)]) ; the variables to update
+  (let ([new-s : State (hash-copy s)])
+    (for ([x xs])
+      (let ([f (hash-ref bn x)])
+        (hash-set! new-s x (f s))))
+    new-s))