nObject</a>
              that can be invoked with <code class="computeroutput">Expr</code>'s tag and children as
              expressions, as follows:
              </p>
<pre class="programlisting"><span class="identifier">context</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">(</span><span class="special">)</span><span class="special">,</span> <a cla