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

theorem
  Rea(z1*z2*z3) = Rea (z3*z1*z2)
proof
  Rea(z1*z2*z3) = Rea(z1*(z2*z3)) by QUATERN2:16
    .=Rea((z2*z3)*z1) by Th1
    .=Rea(z2*(z3*z1)) by QUATERN2:16
    .=Rea (z3*z1*z2) by Th1;
  hence thesis;
end;
