">value_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">value_type</span><span class="special">&amp;</span> <span class="identifier">t</span><span class="special">)</span> <span class="keyword">const</span>
   <span class="special">{</span>
      <span class="comment">// pi * Jn(x) = Int_0^pi [cos(x * sin(t) - n*t) dt]</span>
      <span class="keyword">return</span> <span class="identifier">cos</span><span class="s