title: Scheme - car/cdr on empty list
body: <p>I wrote a big program that use car and cdr, and do:</p>
  
  <pre><code>(map car (append (map caddr lists) (map cadr lists))
  </code></pre>
  
  <p>When <code>lists</code> is list of lists in the next format <code>((a (b) (c)) (d (e) (f))...(x (y) (z)))</code>
  When I did it I got one list <code>(b c e f... y z)</code>
  (Note: b,c,...y,z is a list of numbers; a,d...x is a symbol)</p>
  
  <p>But now, I found that b,c,...,y,z can be also empty list, and It gives the next error:</p>
  
  <pre><code>car: expects argument of type &lt;pair&gt;; given ()
  </code></pre>
  
  <p>What can I do?</p>