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

theorem
  -(z1+z2+z3)=-z1-z2-z3
proof
 -(z1+z2+z3)=-(z1+(z2+z3)) by QUATERN2:2
    .=-z1-(z2+z3) by Th52
    .=-z1-z2-z3 by Th54;
  hence thesis;
end;
