networks: Rewrite update for immutable hashes.

networks.rkt

@@ -63,14 +63,11 @@
 
 ;;; Given a state s updates all the variables from xs.  This
 ;;; corresponds to a parallel mode.
-(define (update n   ; the network
-                s   ; the state to operate on
-                xs) ; the variables to update
-  (let ([new-s (hash-copy s)])
-    (for ([x xs])
-      (let ([f (hash-ref n x)])
-        (hash-set! new-s x (f s))))
-    new-s))
+(define (update network s xs)
+  (for/fold ([new-s s])
+            ([x xs])
+    (let ([f (hash-ref network x)])
+      (hash-set new-s x (f s)))))
 
 ;;; A version of make-immutable-hash restricted to creating network
 ;;; states (see contract).