reserve E, x, y, X for set;
reserve A, B, C for Subset of E^omega;
reserve a, b for Element of E^omega;
reserve i, k, l, kl, m, n, mn for Nat;

theorem
  A c= B implies A? c= B?
proof
  assume A c= B;
  then A |^ (0, 1) c= B |^ (0, 1) by Th29;
  then A? c= B |^ (0, 1) by Th79;
  hence thesis by Th79;
end;
