title: Sum numbers that are positive which is less than n
body: <pre><code>(defun sum (n) 
    (if (n&lt;0) 0 n-1) ;; if n&lt;0, add 0. Else add the next smallest.
    (sum (n-1)))
  </code></pre>
  
  <p>So far I come out with something like this but I am not sure how do I declare a variable to store the sum that I would like to return.</p>