de> and emit the value stored in the
              <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code>.
              Otherwise the generators will fail.
            </p></td></tr>
</table></div>
<h6>
<a name="spirit.karma.reference.string.string.h6"></a>
            <span class="phrase"><a name="spirit.karma.reference.string.string.complexity"></a></span><a class="link" href="st