tifier">impl</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <em class="replaceable"><code><span class="identifier">unspecified</span></code></em><span class="special">,</span> <em class="replaceable"><code><span class="identifier">unspecified</span></code></em> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">result_type</span> <a name="boost.proto.transform.result_1_3_33_5_47_2_1_1_3.type"></a><span class="i