theorem Th22:
  2*`K = K+`K
proof
  thus 2*`K = card([:{{}},K:] \/ [:{1},K:]) by CARD_1:50,ZFMISC_1:109
    .= card([:K,{{}}:] \/ [:K,{1}:]) by Th15
    .= card K +` card K by Th16
    .= K +` card K
    .= K +` K;
end;
