Функция Лапласа В Дельфи

[useryur]

Подскажите, пожалуйста, в чем проблема?
Выдаёт ошибку Error: local procedure/function 'Fun' assigned to procedure variable с переводом курсора на строку result:=Simpson(Fun,0,x,1E-12)/sqrt(2*pi);.

unit Unit1;

interface

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

type
  TFunction = function(x: double): double;
  TForm1 = class(TForm)
    edt1: TEdit;
    edt2: TEdit;
    btn1: TButton;
    procedure btn1Click(Sender: TObject);

  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.btn1Click(Sender: TObject);

function Simpson(Fun: TFunction; a,b,eps: double): double;
var 
i,n: integer; 
h,s0,s: double; 
begin 
n:=1; s:=0; 
repeat 
n:=n*2; h:=(b-a)/n; s0:=s; 
s:=Fun(a)+Fun(b); 
for i:=1 to n-1 do 
s:=s+2*(1+i mod 2)*Fun(a+i*h); 
s:=s*(b-a)/(3*n); 
until abs(s-s0) <= eps; 
result:=s; 
end; //Simpson

function Fun(t: double): double;
begin
result:=exp(-t*t/2); 
end; //Fun 

function F(x: double): double; 
begin 
result:=Simpson(Fun,0,x,1E-12)/sqrt(2*pi);
end; //F

 var
  d,h:Real;
  h_s,x_s:string;
   code:Integer;
begin
 x_s :=edt1.Text;
val(x_s,d,code);
  h:= F(d);
  Str(h:3:6, h_s);
edt2.Text:=h_s;
end;

end.