identifier">optional</span><span class="special">)</span> <span class="identifier">An</span> <span class="identifier">optional</span> <span class="keyword">template</span> <span class="identifier">parameter</span><span class="special">,</span> <span class="identifier">defaulting</span> <span class="identifier">to</span> <span class="identifier">a</span> <span class="identifier">marker</span> <span class="identifier">type</span><span class="special">.</span>
                     <span class="identifier">If</