title: lisp -how to check if all the list is numbers
body: <p>I build this function to check if all the "var" on the list are numbers.
  This what i tried to do</p>
  
  <pre><code>(defun check6 (list) 
  (if  (null list) 'TRUE)
  (if (not (numberp(first list))) nil)
  (check6 (rest list)))
  </code></pre>
  
  <p>But always i get stack overflow.</p>
  
  <p>Why please?</p>