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

theorem
  |.z1 - z2.| <= |.z1 - z.| + |.z - z2.|
proof
  |.z1 - z2.| = |.(z1 - z) + (z - z2).|;
  hence thesis by Th56;
end;
