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? = (A \ {<%>E})?
proof
  thus A? = {<%>E} \/ A by Th76
    .= {<%>E} \/ (A \ {<%>E}) by XBOOLE_1:39
    .= (A \ {<%>E})? by Th76;
end;
