span> <span class="keyword"></span> mpl::bool_&lt;<em class="replaceable"><code>true-or-false</code></em>&gt; <span class="special">{</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id-1.3.33.5.41.18.4"></a><h2>Description</h2>
<p>
            <code class="computeroutput">proto::is_callable&lt;&gt;</code> is used by the
            <code class="computeroutput"><a class="link" href="when.html" title="Struct template when">proto::when&lt;&gt;</a>