title: Inserting an element at given position in list
body: <p>I don't know how I can append an element in n position in a list.
  For example:</p>
  
  <pre><code>(insert-at new k lis)
  (insert-at ’N 2 ’(a b c d e f))
  =&gt;
  ’(a b N c d e f)
  </code></pre>
  
  <p>It is ok?:</p>
  
  <pre><code>(define (insert-at new k lis)
    (cond (( null? lis)
           (list new))   
          (zero? k   
           (cons new lis))   
          (else       
           (cons (car lis)
                 (insert-at new (- k 1) (cdr lis))))))
  </code></pre>