e="Concept Expr">Expr</a><span class="special">,</span> <span class="keyword">typename</span> State<span class="special">,</span> <span class="keyword">typename</span> Data<span class="special">&gt;</span> 
<span class="keyword">struct</span> <a class="link" href="impl.html" title="Struct template impl">impl</a> <span class="special">:</span>
  <span class="keyword"></span> <a class="link" href="../pass_through.html" title="Struct template pass_through">proto::pass_through</a>&lt;shift_left&gt;::template im