    semantics for the attribute. For this reason the attribute type needs
              to to be usable with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code>
              (needs to either define a proper overload for <code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">attribute_type</span><span class="special">&amp;,</span> <span class="identifier">