reserve x,O for set,
  o for Element of O,
  G,H,I for GroupWithOperators of O,
  A, B for Subset of G,
  N for normal StableSubgroup of G,
  H1,H2,H3 for StableSubgroup of G,
  g1,g2 for Element of G,
  h1,h2 for Element of H1,
  h for Homomorphism of G,H;

theorem Th28:
  for N1,N2 being strict normal StableSubgroup of G holds N1 * N2 = N2 * N1
proof
  let N1,N2 be strict normal StableSubgroup of G;
  reconsider N19= the multMagma of N1,N29= the multMagma of N2 as strict
  normal Subgroup of G by Lm6;
  thus N1 * N2 = carr N29 * carr N19 by GROUP_3:125
    .= N2 * N1;
end;
