this composite as a new component somewhere else.
  <li>Efficient, so we're not required to re-implement it again.
  <li>Scalable, so that it works well for short and arbitrarily long sequences.
</ul>
The approach that comes closest to meeting these requirements is that described
and implemented with <a href="../../iterator/doc/index.html">Iterator Adaptors</a>.
The fundamental feature of an Iterator Adaptor template that makes in interesting to
us is that it takes as a parameter a base iterator from which 