theorem Th9:
  x1 <> x2 implies K+`M,[:K,{x1}:] \/ [:M,{x2}:] are_equipotent &
  K+`M = card([:K,{x1}:] \/ [:M,{x2}:])
proof
  assume x1 <> x2;
  then card([:K,{x1}:] \/ [:M,{x2}:]) = K+`M by Lm1;
  hence thesis by CARD_1:def 2;
end;
