title: Clojure: How to generate a 'trie'?
body: <p>Given the following...</p>
  
  <pre><code>(def inTree
   '((1 2)
     (1 2 3)
     (1 2 4 5 9)
     (1 2 4 10 15)
     (1 2 4 20 25)))
  </code></pre>
  
  <p>How would you transform it to this trie?</p>
  
  <pre><code>(def outTrie
   '(1
      (2 ()
         (3 ())
         (4 (5
              (9 ()))
            (10
              (15 ()))
            (20
              (25 ()))))))
  </code></pre>