 which is an opaque record wrapped around a list continuation.
   These iterators are then used to provide a version of the stdlib
   <code>lists</code> functions which only evaluate elements of the iterator   
when demanded.</p>
  
   <p>Several simple iterator constructors are provided as well as a
   general purpose <code>unfold/2</code> constructor. Conversion to built in
   types for lists and maps as the <code>from_list/1</code>, <code>to_list/1</code>,
   <code>from_map/1</code> and <code>to_map/1</c