Add some background knowledge to the recursion section.

src/koans/recursion.clj

@@ -1,7 +1,25 @@
+(defn is-even? [n]
+  (if (= n 0) __
+      (___ (is-even? (dec n)))))
+
+(defn is-even-bigint? [n]
+  (loop [n n acc true]
+    (if (= n 0) __
+        (recur (dec n) (not acc)))))
+
 (defn factorial [n]
   __)
 
 (meditations
+  "Recursion ends with a base case"
+  (= true (is-even? 0))
+
+  "And starts by moving toward that base case"
+  (= false (is-even? 1))
+
+  "Having too many stack frames requires explicit tail calls with recur"
+  (= false (is-even-bigint? 100003N))
+
   "Simple things may appear simple."
   (= 1 (factorial 1))