                                                  <a name="boost.signals2.signal.argument_type"></a><span class="identifier">argument_type</span><span class="special">;</span>                <span class="comment">// Exists iff arity == 1</span>
  <span class="keyword">typedef</span> <span class="identifier">T1</span>                                                                                 <a name="boost.signals2.signal.first_argument_type"></a><span class="identifier">first_argument_type</span><span 