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 c= B+ implies B+ = (B \/ A)+
proof
  assume A c= B+;
  then A c= B |^.. 1 by Th50;
  then B |^.. 1 = (B \/ A) |^.. 1 by Th47;
  then B |^.. 1 = (B \/ A)+ by Th50;
  hence thesis by Th50;
end;
