n class="special">=</span> <span class="identifier">finite_difference_derivative</span><span class="special">&lt;</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">f</span><span class="special">),</span> <span class="identifier">Real</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;(</span><span class="identifier">f</span><span class="special">,</span> <span class="identifier">x</span><span class="special">);</span>
<