Refs. We has them.

src/koans/refs.clj

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+(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)))