an><span class="special">)</span>
   <span class="special">{</span>
      <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">sqrt</span><span class="special">;</span>
      <span class="comment">// Return the difference between required arc-length, and the calculated arc-length for an</span>
      <span class="comment">// ellipse with radii m_radius and x, plus it's derivative.</span>
      <span class="comment">// See http://www.wol