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;
