reserve X for non empty 1-sorted;

theorem Th1:
  for A, B being Subset of X holds ( A`) \ B` = B \ A
proof
  let A, B be Subset of X;
  A` \ B` = [#]X \ (A \/ B`) by XBOOLE_1:41;
  then A` \ B` = ( A`) /\ B`` by XBOOLE_1:53;
  hence thesis by SUBSET_1:13;
end;
