theorem Th60:
  (for g holds g in H1 iff g in H2) implies the addMagma of H1 =
  the addMagma of H2
proof
  assume for g holds g in H1 iff g in H2;
  then H1 is Subgroup of H2 & H2 is Subgroup of H1 by Th58;
  hence thesis by Th55;
end;
