Add recursion / factorial koan.

src/koans/recursion.clj

@@ -0,0 +1,21 @@
+(defn factorial [n]
+  __)
+
+(meditations
+  "Simple things may appear simple."
+  (= 1 (factorial 1))
+
+  "They may require other simple steps."
+  (= 2 (factorial 2))
+
+  "Sometimes a slightly bigger step is necessary"
+  (= 6 (factorial 3))
+
+  "And eventually you must think harder"
+  (= 24 (factorial 4))
+
+  "You can even deal with very large numbers"
+  (= (factorial 1000N) (factorial 1000N))
+
+  "But what happens when the machine limits you?"
+  (= (factorial 10000N) (factorial 10000N)))

src/path_to_answer_sheet.clj

@@ -87,6 +87,10 @@
                               :a :b :c :d
                               :c :d]
                         "___" ["multiply-by-5"]}
+   "factorial" {"__" ["(loop [n n acc 1]
+                         (if (zero? n)
+                             acc
+                             (recur (dec n) (* acc n))))"]}
    })
 
 (defn replace-with [s k replacements]

src/path_to_enlightenment.clj

@@ -25,7 +25,8 @@
       "higher_order_functions"
       "runtime_polymorphism"
       "sequence_comprehensions"
-      "partial_functions"])
+      "partial_functions"
+      "recursion"])
 
 (defn run []
   (apply load (doall (map (partial str "koans/") ordered-koans)))