ep copy converts the proto tree to hold this by value. The deep copy
          operation can be left out for simpler proto expressions (not containing
          references to temporaries). Alternatively you could use the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span></code> facility to build the required
          proto expression.
        </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">namespace</spa