<span class="keyword">const</span> <a class="link" href="../boost/histogram/axis/variant.html" title="Class template variant">variant</a><span class="special">&lt;</span> <span class="identifier">Ts</span><span class="special">...</span> <span class="special">&gt;</span> <span class="special">&amp;</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
        <sp