Refs. We has them.

src/koans/refs.clj

@@ -0,0 +1,27 @@
+(def the-world (ref "hello"))
+
+(meditations
+  "In the beginning, there was a word"
+  (= __ (deref the-world))
+
+  "You can get the word more succinctly, but it's the same"
+  (= __ @the-world)
+
+  "You can be the change you wish to see in the world."
+  (= __ (do
+          (dosync (ref-set the-world "better"))
+          @the-world))
+
+  "Alter where you need not replace"
+  (= __ (let [exclamator (fn [x] (str x "!"))]
+          (dosync
+            (alter the-world exclamator)
+            (alter the-world exclamator)
+            (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)))

src/path_to_answer_sheet.clj

@@ -111,6 +111,12 @@
                                  (repeat " "))))
                           '{:original-parts full-name
                             :named-parts {:first first-name :last last-name}} ]}
+   "refs" {"__" ["\"hello\""
+                 "\"hello\""
+                 "\"better\""
+                 "\"better!!!\""
+                 ]
+           "___" ['(comp exclamator exclamator exclamator)]}
    })
 
 (defn replace-with [s k replacements]

src/path_to_enlightenment.clj

@@ -27,7 +27,8 @@
       "sequence_comprehensions"
       "partial_functions"
       "recursion"
-      "destructuring"])
+      "destructuring"
+      "refs"])
 
 (defn run []
   (apply load (doall (map (partial str "koans/") ordered-koans)))