vative.</span>
      <span class="comment">// See http://www.wolframalpha.com/input/?i=d%2Fda+[4+*+a+*+EllipticE%281+-+b^2%2Fa^2%29]</span>
      <span class="comment">// We require two elliptic integral calls, but from these we can calculate both</span>
      <span class="comment">// the function and it's derivative:</span>
      <span class="identifier">T</span> <span class="identifier">a</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">std</span><span class="sp