h iterators.</p>
  
   <p>The interface for this module attempts to follow the <code>lists</code>   
behaviour as closely as possible. Guidelines for how past and   
future translation is performed is as follows:</p>
  
   <ul>
   <li>Any input lists are changed to expect iterators.</li>
   <li>Any output lists are changed to be iterators.</li>
   <li>Elements of input iterators should be evaluated exactly
   once per output iterator.</li>
   <li>Any numeric counts for repetition are changed to allow
   'in