theorem Th57:
  |.z1 - z2.| <= |.z1.| + |.z2.|
proof
  |.z1 - z2.| = |.z1 + - z2.|;
  then |.z1 - z2.| <= |.z1.| + |.-z2.| by Th56;
  hence thesis by Lm26;
end;
