oward Hinnant have submitted a "compressed_pair"
          library to Boost, which uses a technique similar to the one described here
          to hold references. Their pair also uses type traits to determine if any
          of the types are empty, and will derive instead of contain to conserve
          space -- hence the name "compressed".
        </li>
<li class="listitem">
          This is actually an issue with the C++ Core Language Working Group (issue
          #106), submitted by Bjarne Stroustru