type TSelection = class(TList)
public
  procedure Check(const Id : Integer);
  function Checked(const Id : Integer) : boolean;
  function Find(const Id : Integer) : Integer;
  destructor Destroy; override;
  procedure ClearAll;
end;

procedure TSelection.Check(const Id: Integer);
var i : integer;
    Pt : PtInt;
begin
  i := Find(Id);
  if i = - 1  then begin
    New(Pt);
    Pt^ := Id;
    Add(Pt);
  end
  else begin
    Dispose(Items[i]);
    Delete(i);
  end;
end;

procedure TSelection.ClearAll;
var i : integer;
begin
  for i :=  0  to Pred(Count) do
    Dispose(Items[i]);
  Clear;
end;

destructor TSelection.Destroy;
var i : integer;
begin
  for i :=  0  to Pred(Count) do
    Dispose(Items[i]);
  inherited;
end;

function TSelection.Find(const Id: Integer): Integer;
var i : integer;
begin
  Result := - 1 ;
  i :=  0 ;
  while (i < Count) and (PtInt(Items[i])^ <> Id) do
    Inc(i);
  if i < Count then
    Result := i;
end;

function TSelection.Checked(const Id: Integer): boolean;
begin
  Result := Find(Id) <> - 1 ;
end;