reserve X1,X2,X3,X4 for set;

theorem Thm04:
  (X1 \ X2) \ (X3 \ X4) = ((X1 /\ X4) \ (X2 \/ X3)) \/
  (X1 \ (X2 \/ X3 \/ X4)) \/ ((X1 /\ X3 /\ X4) \ X2)
  proof
    (X1 \ X2) \ (X3 \ X4) = (X1 \ (X2 \/ X3)) \/ ((X1 /\ X4) \ X2) by Thm02;
    hence thesis by Thm03;
  end;
