Почему выдает ошибку undeclared identifier checkbox?

unit Unit1;
interface
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    CheckBox1: TCheckBox;
    CheckBox2: TCheckBox;
    CheckBox3: TCheckBox;
    CheckBox4: TCheckBox;
    CheckBox5: TCheckBox;
    lbl1: TLabel;
    procedure FormCreate(Sender: TObject);
    procedure CheckBox1Click(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation
{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
var s: Boolean;
begin
  s:=False;
end;

procedure TForm1.CheckBox1Click(Sender: TObject);
  var
    Index, i, num: Integer;
    e, s: Boolean;
begin
  if s then
    {Åñëè ïðîãðàìììà ñíîâà âûçîâåò ïðîöåäóðó îáðàáîòêè,
     áóäåò âûïîëíåí îïåðàòîð Exit - íåìåäëåííûóé âûõîä èç ïðîöåäóðû}
    Exit;
  s:=True;
  for Index:=0 to 4 do //Îïåðàòîð break ïðåðûâàåò
                       //âûïîëíåíèå öèêëà
    if Sender = Controls[i] then
      break; //Îïðåäåëÿåòñÿ, êàêîé ôëàæîê áûë ïåðåêëþ÷åí
  {Êîãäà âûïîëíåíèå öèêëà çàâåðøàåòñÿ, çíà÷åíèå ïåðåìåííîé Index
   ñîîòâåòñòâóåò ïåðåêëþ÷åííîìó ôëàæêó}
  {Åñëè çíà÷åíèå ñâîéñòâà Checked (Óñòàíîâëåí) ðàâíî False (Íåò),
   ôëàæîê ñáðîøåí, à åñëè True (Äà) - óñòàíîâëåí.
    Íîìåð ôëàæêà â ìàññèâå îïðåäåëÿòåñÿ ïåðåìåííîé Index}
   if not(Controls[Index] as TCheckBox).Checked then
    {Óñëîâèå âûïîëåíî, åñëè ôëàæîê ñåé÷àñ ñáðîøåí,
     ò.å. äî çåë÷êà îí áûë óñòàíîâëåí}
   (Controls[Index] as TCheckBox).Checked:=True
   {Ïðîãðàììèðîâàíèå èçìåíåíèÿ ñîñòîÿíèÿ "äîïîëíèòåëüíûõ
   ôëàæêîâ". Òåêóùèé ôëàæîê óæå ïåðåêëþ÷åí}
   else
   begin
     num:=Index+1; //Ïåðåêëþ÷åíèå äâóõ ñëåäóþùèõ ôëàæêîâ
    if Index = 4 then
      num:=0; //Èçìåíåíèå ñîñòîÿíèÿ íîâîãî ôëàæêà
    //Ñîñòîÿíèå ôëàæêà íàäî ïîìåíÿòü íà ïðîòèâîïîëîæíîå
    (Controls[num] as TCheckBox).Checked:=
      not(Controls[num] as CheckBox).Checked;
    //Âûïîëíåíèå ïðîâåðêè íà âûõîä çà ïðåäåëû ìàññèâà
    num:=num+1;
    if Index = 3 then
      num:=0;
    (Controls[num] as TCheckBox).Checked:=
      not(Controls[num] as CheckBox).Checked;
   end;
   //Ãîëîâîëîìêà ðåøåíà, åñëè óñòàíîâëåíû âñå ïÿòü ôëàæêîâ
   e:=True;
   for i:= 0 to 4 do
    e:=e and (Controls[i] as TCheckBox).Checked;
    {Ïîñëå öèêëà çíà÷åíèå îñòàíåòñÿ ðàâíûì True,
     åñëè âñå ôëàæêè óñòàíîâëåíû}
   if e then //Ãîëîâîëîìêà ðåøåíà
    for i:=0 to 4 do
    begin
      (Controls[i] as TCheckBox).Caption:='Ïîáåäà';
      (Controls[i] as TCheckBox).Enabled:=False; //Îòêëþ÷åíèå ôëàæêîâ
    end;
    s:=False;
end;

end.