 complex function evaluation and is not subject to the catastrophic subtractive
      cancellation that plagues finite-difference calculations.
    </p>
<p>
      An example usage:
    </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">7.2</span><span class="special">;</span>
<span class="keyword">double</span> <span class="identifier">e_prime</span> <span class="special">=</span> <span class="identifier"