theorem Th21:
  (1).G /\ H1 = (1).G & H1 /\ (1).G = (1).G
proof
A1: (1).G is StableSubgroup of H1 by Th16;
  hence (1).G /\ H1 = (1).G by Lm21;
  thus thesis by A1,Lm21;
end;
