theorem
  for N1,N2 being strict normal Subgroup of G holds N1 /\ N2 is normal
proof
  let N1,N2 be strict normal Subgroup of G;
  let a;
  thus (N1 /\ N2) * a = (N1 * a) /\ (N2 * a) by Th63
    .= N1 /\ (N2 * a) by Def13
    .= the addMagma of (N1 /\ N2) by Def13;
end;
