title: What are circular lists good for (in Lisp or Scheme)?
body: <p>I note that Scheme and Lisp (I guess) support circular lists, and I have used circular lists in C/C++ to 'simplify' the insertion and deletion of elements, but what are they good for?</p>
  
  <p>Scheme ensures that they can be built and processed, but for what?</p>
  
  <p>Is there a 'killer' data structure that needs to be circular or tail-circular?</p>