pan class="k">typedef</span> <span class="kt">double</span> <span class="n">second_argument_type</span><span class="p">;</span>
  <span class="k">typedef</span> <span class="kt">double</span> <span class="n">result_type</span><span class="p">;</span>

  <span class="kt">double</span> <span class="nf">operator</span><span class="p">()(</span><span class="kt">double</span> <span class="n">a</span><span class="p">,</span><span class="kt">double</span> <span class="n">b</span><span class="p">)</span> <span clas