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 Th58:
  A c= B implies A+ c= B+
proof
  assume A c= B;
  then A |^.. 1 c= B |^.. 1 by Th13;
  then A+ c= B |^.. 1 by Th50;
  hence thesis by Th50;
end;
