template<typename Signature,   // Function type R (T1, T2, ..., TN)
         typename Combiner = last_value<R>, 
         typename Group = int, 
         typename GroupCompare = std::less<Group>, 
         typename SlotFunction = functionN<Signature> > 
class signal : public signalN<R, T1, T2, ..., TN, Combiner, Group, GroupCompare, SlotFunction>
{
public:
  // construct/copy/destruct
  signal(const combiner_type& = combiner_type(), 
         const group_compare_type& = group_compare_type());
};