theorem Th39:
  for X,Y being finite set st X misses Y holds card (X \/ Y) = card X + card Y
proof
  let X,Y be finite set;
  assume X misses Y;
  then card card (X \/ Y) = card card X +` card card Y by Th34
    .= card (card X + card Y) by Th37;
  hence thesis;
end;
