theorem Th21:
  B+^C = B+^D implies C = D
proof
  assume that
A1: B+^C = B+^D and
A2: C <> D;
  C in D or D in C by A2,ORDINAL1:14;
  then B+^C in B+^C by A1,ORDINAL2:32;
  hence contradiction;
end;
