theorem Th8:
  for T being 1-sorted, F being Subset-Family of T holds
  COMPLEMENT(F) is finite iff F is finite
proof
  let T be 1-sorted, F be Subset-Family of T;
  thus COMPLEMENT(F) is finite implies F is finite by Lm1;
  assume F is finite;
  then COMPLEMENT COMPLEMENT F is finite;
  hence thesis by Lm1;
end;
