t</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
  <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Visitor<span class="special">&gt;</span> <span class="keyword">void</span> <a class="link" href="nil.html#idp60416544-bb"><span class="identifier">apply_to</span></a><span class="special">(</span><span class="identifier">Visitor</span> <span class="special">&amp;</span><span class="special">)</spa