reserve A, B, X, Y for set;

theorem Th2:
  (delta X)"[:A,A:] c= A
proof
  set f = delta X;
  let x be object;
  assume x in f"[:A,A:];
  then f.x in [:A,A:] & f.x = [x,x] by FUNCT_1:def 7,FUNCT_3:def 6;
  hence thesis by ZFMISC_1:87;
end;
