void List::insert(int value)
{
     ListCell *pt = new ListCell( value, head );

     // this line added to handle tail
     if ( head == 0 ) tail = pt;

     head = pt;
     theCount++;
}

void List::append( int value )
{
     ListCell *pt = new ListCell( value );
     if ( head == 0 )
          head = pt;
     else
          tail->next = pt;
     
     tail = pt;
     theCount++;
}

int List::is_present( int value ) const
{
     if ( head == 0 ) return 0;
     if ( head->val == value || tail->val == value )
          return 1;
     
     ListCell *pt = head->next;
     for (; pt != tail; pt = pt->next)
          if ( pt->val == value )
               return 1;
          
     return 0;
}