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? = {<%>E} iff A = {} or A = {<%>E}
proof
  A? = A |^ (0, 1) by Th79;
  hence thesis by Th31;
end;
