n class="identifier">none_t</span> <a class="link" href="../boost/histogram/axis/option/none.html" title="Global none">none</a><span class="special">;</span>        <span class="comment">// Instance of <code class="computeroutput">none_t</code>. </span>
        <span class="keyword">constexpr</span> <span class="identifier">underflow_t</span> <a class="link" href="../boost/histogram/axis/option/underflow.html" title="Global underflow">underflow</a><span class="special">;</span>        <span class="comment">