entifier">result_of</span><span class="special">&lt;</span>
    <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="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class