reserve X for set;

theorem
  for A,B being Subset of X holds {A,B} is N_Sub_set_fam of X
proof
  let A,B be Subset of X;
  ex F being sequence of bool X st (rng F = {A,B} & F.0 = A & for n
  being Nat st 0 < n holds F.n = B) by Th19;
  hence thesis by SUPINF_2:def 8;
end;
