o their usual attribute of type <code class="computeroutput"><span class="identifier">Num</span></code>
              all listed generators accept an instance of a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">Num</span><span class="special">&gt;</span></code> as well. If the <code class="computeroutput"><span class="identifier">boost</span><span class="special