lfill this requirement.  When using <code class="computeroutput"><a class="link" href="../boost/signals2/optional_last_value.html" title="Class template optional_last_value">optional_last_value</a></code>,
      there is no requirement for slots to be connected when a signal
      is invoked, since in that case the combiner may simply return an empty
      <code class="computeroutput">boost::optional</code>.
    </p>
</div>
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