ss="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">S</span><span class="special">&gt;</span></code> as well. If 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> is initialized (holds a value)
              the generators behave as if their attribute was an instance of 