theorem Th93:
  M c= M+`N & N c= M+`N
proof
A1: M c= M +^ N by ORDINAL3:24;
A2: N c= M +^ N by ORDINAL3:24;
A3: card N = N;
  card M = M;
  then
A4: M c= card ( M +^ N) by A1,CARD_1:11;
  N c= card ( M +^ N) by A2,A3,CARD_1:11;
  hence thesis by A4;
end;
