theorem Th104:
  |.z1 - z2.| <= |.z1 - z.| + |.z - z2.|
proof
  |.z1 - z2.| = |.z1 - z + z - z2.| by Th80
    .= |.(z1 - z) + (z - z2).| by Th75;
  hence thesis by Th97;
end;
