title: Help using Lisp debugger
body: <p>I'm trying understand how to interpret the output of, and use, the Lisp debugger.</p>
  
  <p>I've got a pretty simple Backtrace for the evaluation of my function, but I cann't seem to work out how to use it to find out in which Lisp 'form' in my function the exception occurred.</p>
  
  <p>I'd appreciate any clues as to what I should be doing, to find where in my code the error originated.</p>
  
  <p>Also - why does the second frame display as "no debug information available for frame"?</p>
  
  <p>I've attached a screen shot with the debugger, and repl  (I've also included my function below - I know it's very wrong - but I'm just interested in learning to use the debugger properly). In addition, I hit 'v' on the first frame to go to the source, but this resulted in the error below the repl. (EDIT - the missing source code issue is fixed by downloading &amp; copying it to the correct path)</p>
  
  <p><img src="https://i.stack.imgur.com/PqKJv.png" alt="alt text"></p>
  
  <p>(horrible function - no comments please!)</p>
  
  <pre><code>(defun myquicksort2 (lst)
    (if (eql 1 (length lst))
        lst
        (let ((mid (middle lst)))
      (do ((i 0 (+ i 1)))
          ((&gt;= i mid) (append (myquicksort2 (subseq lst 0 mid))
                    (myquicksort2 (subseq lst mid (length lst)))))
        (if (&gt; (ltval i lst) (nth 100 lst))
            (let ((tmp (ltval i lst)))
          (setf (nth i lst) (gtval i lst))
          (setf (nth (- (- (length lst) i) 1) lst)  tmp)))))))
  
  (defun ltval (i lst)
    (nth i lst))
  
  (defun gtval (i lst)
    (nth (- (- (length lst) i) 1) lst))
  
  (defun middle (lst)
    (round (/ (length lst) 2)))
  </code></pre>