, this is a reference. Hence, there is no way to know the
            exact element type using <code class="computeroutput"><a class="link" href="../intrinsic/metafunctions/at.html" title="at"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at</span></code></a><span class="special">&lt;</span><span class="identifier">S</span><span class="special">,</span> <span class="identifier">M</span><span class="special">&gt;</span></code>.Th