theorem ThD1Commutative: :: TH111
  Dihedral_group 1, INT.Group 2 are_isomorphic
  & Dihedral_group 1 is commutative
proof
  thus Dihedral_group 1, INT.Group 2 are_isomorphic by Th73;
  hence Dihedral_group 1 is commutative by GROUP_6:77;
end;
