title: Lisp unbound symbol
body: <p>I am learning lisp and i am trying to create simple function to sum arr.</p>
  
  <pre><code>(defun sum( N )
      ( if ( null N )
           nil
           ( + (car N ) ( sum ( cdr N )))))
  
  (sum '(1 2 3 ))
  </code></pre>
  
  <p>yet it throws error </p>
  
  <blockquote>
    <p>Error: execute: unbound symbol: "N" []</p>
  </blockquote>
  
  <p>The parenthesis should be correct. Why is this throwing the error or what is wrong with logic behind my code? </p>
  
  <p>If list is empty it will return nil , if not , it will return first element + rest of the list recursively.</p>
  
  <p>Thanks for help</p>