title: walk/postwalk usage
body: <p>I am going through this <a href="http://www.ibm.com/developerworks/java/library/j-treevisit/index.html?ca=drs-" rel="nofollow">article on Tree Visitors in Clojure</a> and came across the below example:</p>
  
  <pre><code>(def data [[1 :foo] [2 [3 [4 "abc"]] 5]])
  
  (walk/postwalk #(do (println "visiting:" %) %) data)
  </code></pre>
  
  <p>What is the outer form of postwalk doing? I can't understand its utility. How and why is postwalk used? Any explanations will be appreciated.</p>