class="special">;</span>          <span class="comment">// Exists iff arity == 2</span>
  <span class="keyword">typedef</span> <span class="identifier">T2</span>                                                                                 <a name="boost.signals2.signal.second_argument_type"></a><span class="identifier">second_argument_type</span><span class="special">;</span>         <span class="comment">// Exists iff arity == 2</span>

  <span class="comment">// static constants</span>
  <span class="k