reserve X for set;

theorem Th24:
  for S being SigmaField of X, F being sequence of S holds
  union rng F is Element of S
proof
  let S be SigmaField of X, F be sequence of S;
  rng F is N_Sub_set_fam of X & rng F c= S by Th23,RELAT_1:def 19;
  hence thesis by Def5;
end;
