theorem
  |.z1.| - |.z2.| <= |.z1 - z2.|
proof
  z1 = z1 - z2 + z2 by Th80;
  then |.z1.| <= |.z1 - z2.| + |.z2.| by Th97;
  hence thesis by XREAL_1:20;
end;
