is differs from
      the original Boost.Signals library, which instead relied on derivation
      from the <code class="computeroutput">boost::signals::trackable</code> class.
      The library would be
      notified of an object's destruction by the
      <code class="computeroutput">boost::signals::trackable</code> destructor.
    </p>
<p>Unfortunately, the <code class="computeroutput">boost::signals::trackable</code>
      scheme cannot be made thread safe due
      to destructor ordering.  The destruc