title: Get last n elements in a list in DrRacket, without list-ref
body: <p>I know how to get the first <code>n</code> elements in a list, </p>
  
  <pre><code>(define (countup n ls)
    (cond
      [(zero? n) '()]
      [else (cons (first ls) (countup (sub1 n) (rest ls)))]))
  </code></pre>
  
  <p>but how can I do something like this for the <em>last</em> <code>n</code> elements in a list <em>(without using list-ref)</em>?</p>
  
  <p>If I call <code>(countup 3 '(a b c d e))</code>, I get <code>(list a b c)</code>. I need to be able to enter <code>(counter 3 '(a b c d e))</code> and get <code>(list c d e)</code>.</p>
  
  <p>I need error messages in case the number <code>n</code> given is bigger than the list's length.</p>