reserve E, x, y, X for set;
reserve A, B, C for Subset of E^omega;
reserve a, a1, a2, b for Element of E^omega;
reserve i, k, l, m, n for Nat;

theorem
  A+ = {} iff A = {}
proof
  A+ = A |^.. 1 by Th50;
  hence thesis by Th4;
end;
