theorem
  A c= B+ & n > 0 implies A |^ n c= B+
proof
  assume that
A1: A c= B+ and
A2: n > 0;
  A c= B |^.. 1 by A1,Th50;
  then A |^ n c= B |^.. 1 by A2,Th28;
  hence thesis by Th50;
end;
