请问这是什么问题？

unit Unit1;interfaceuses
 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, FileCtrl, ExtCtrls, XPMan;type
  TForm1 = class(TForm)
    GroupBox1: TGroupBox;
    GroupBox2: TGroupBox;
    GroupBox3: TGroupBox;
    DriveComboBox1: TDriveComboBox;
    DirectoryListBox1: TDirectoryListBox;
    FileListBox1: TFileListBox;
    Label1: TLabel;
    Edit1: TEdit;
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    procedure Button3Click(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
    function change(souces,desc:string):boolean;
  public    { Public declarations }
  end;var
  Form1: TForm1;implementation{$R *.dfm}
 function change(souces,desc:string):boolean;
 var
  i:integer;
  s:string;
  desstr,sourstr:array [0..255] of char;
 begin
  i:=0;
  result:=false;
  if (trim(desc)<>'') and (trim(souces)<>'')then
  begin
   i:=pos(string(strrscan(pchar(souces),'\')),souces);
   s:=copy(souces,0,i);
   chdir(s);
   s:=s+desc;
    strcopy(@sourstr,pchar(souces));
    strcopy(@desstr,pchar(s));
    if movefile(@sourstr,@desstr) then
      result:=true;
  end;
  end;procedure TForm1.Button3Click(Sender: TObject);
begin
close;
end;procedure TForm1.Button1Click(Sender: TObject);
begin
if change(directorylistbox1.Directory,edit1.Text) then
begin
 showmessage('修改目录名成功！！');
    DirectoryListbox1.Update;
end;
end;procedure TForm1.Button2Click(Sender: TObject);
begin
if Change(filelistbox1.FileName,edit1.Text) then
  begin
    showmessage('修改文件名成功！！');
    FileListbox1.Update;
  end
  else
    showmessage('修改文件名时出错！！请查看新名称是否正确！');
end;procedure TForm1.FormCreate(Sender: TObject);
begin
DriveComboBox1.Drive:='c';
end;end.
[Error] Unit1.pas(28): Unsatisfied forward or external declaration: 'TForm1.change'
望高手解答。在此先谢过