reserve a,b,c,d for Real;
reserve z,z1,z2 for Complex;

theorem
  |.z1.| - |.z2.| <= |.z1 + z2.|
proof
  z1 = z1 + z2 - z2;
  then |.z1.| <= |.z1 + z2.| + |.z2.| by Th57;
  hence thesis by XREAL_1:20;
end;
