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 B? = (B \/ A)?
proof
  assume
A1: A c= B?;
  thus (B \/ A)? = {<%>E} \/ (B \/ A) by Th76
    .= ({<%>E} \/ B) \/ A by XBOOLE_1:4
    .= B? \/ A by Th76
    .= B? by A1,XBOOLE_1:12;
end;
