#pas
...
function Factorial(n: integer): Longint; //This function for calculating of factorial
var
  f: LongInt;
  i: Integer;
begin
  f := 1;
  for i := 2 to n do
    f := f * i;
  Result := f;
end;

procedure THIMath._work_doOperation44;{fact}
begin
  Res := Factorial(ToInteger(ReadReal(_Data,_data_Op1,_prop_Op1)));
  _hi_OnEvent(_event_onResult,Res);
end;
...