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

theorem
  |.|.z1.| - |.z2.|.| <= |.z2 - z1.|
proof
  |.|.z1.| - |.z2.|.| <= |.z1 - z2.| by QUATERNI:86;
  hence thesis by QUATERNI:83;
end;
