reserve z1,z2,z3,z4,z for Quaternion;

theorem
  -(z1-z2-z3)=-z1+z2+z3
proof
 -(z1-z2-z3)=-(z1-z2)+z3 by Th53
    .=-z1+z2+z3 by Th53;
  hence thesis;
end;
