Provide example of coordinated ref update.

src/koans/refs.clj

@@ -1,4 +1,5 @@
 (def the-world (ref "hello"))
+(def bizarro-world (ref {}))
 
 (meditations
   "In the beginning, there was a word"
@@ -20,16 +21,20 @@
             (alter the-world exclamator))
           @the-world))
 
-  "Though you should keep transactions as short as possible"
-  (= "better!!!!!!" (letfn [(exclamator [x] (str x "!"))]
-          (dosync
-            (alter the-world ___))
-          @the-world))
-
   "Don't forget to do your work in a transaction!"
   (= 0 (do __
          @the-world))
 
   "Functions passed to alter may depend on the data in the ref"
   (= 20 (do
-          (dosync (alter the-world ___)))))
+          (dosync (alter the-world ___))))
+
+  "Two worlds are better than one"
+  (= ["Real Jerry" "Bizarro Jerry"]
+     (do
+       (dosync
+         (ref-set the-world {})
+         (alter the-world assoc :jerry "Real Jerry")
+         (alter bizarro-world assoc :jerry "Bizarro Jerry")
+         __))))
+