theorem
  for N1,N2 being strict normal Subgroup of G holds N1 "\/" N2 is normal
  Subgroup of G
proof
  let N1,N2 be strict normal Subgroup of G;
  (ex N being strict normal Subgroup of G st the carrier of N = carr N1 *
  carr N2 )& the carrier of N1 "\/" N2 = N1 * N2 by Th53,GROUP_3:126;
  hence thesis by GROUP_2:59;
end;
