Operator<<

 ostream&  operator<<(ostream  &os,
                                      const  List  &list)  {
         List::ConstIterator  b  =  list.begin(),
                                        e  =  list.end();
         if  (b  !=  e)  {
                  os  <<  *b;
                  ++b;
         }
         for  (;  b  !=  e;  ++b)
                  os   <<   '   '   <<   *b;
         return  os;
 }