procedure TMainForm.btnMergeClick(Sender: TObject);
begin
   if not CheckIfNonDescendingAndCorrectSG(sgA) then
      MessageBox(0, PChar('List A must be NONdecreasing'), PChar('Warning!'),
        MB_ICONERROR)
   else if not CheckIfNonDescendingAndCorrectSG(sgB) then
      MessageBox(0, PChar('List B must be NONdecreasing'), PChar('Warning!'),
        MB_ICONERROR)
   else
   begin
      ListA := TList.Create;
      ListB := TList.Create;
      ListC := TList.Create;
      try
         ListA.AddFromSG(sgA);
         ListB.AddFromSG(sgB);
      except
         MessageBox(0, PChar('List shouldn''t contain empty fields.'),
           PChar('Warning!'), MB_ICONERROR);
         Exit;
      end;
      ListC := MergeNonDescendingLists(ListA, ListB);
      ListC.Show(sgC);
      ShowOutputElements;
      ListA.Destroy;
      ListB.Destroy;
      ListC.Destroy;
   end;
end;