reserve X,x,y,z for set;
reserve n,m,k,k9,d9 for Nat;
reserve d for non zero Nat;
reserve i,i0,i1 for Element of Seg d;

theorem Th6:
  for X,Y being finite set st X misses Y holds
  (card X is even iff card Y is even) iff card(X \/ Y) is even
proof
  let X,Y be finite set;
  assume X misses Y;
  then card(X \/ Y) = card X + card Y by CARD_2:40;
  hence thesis;
end;
