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

theorem Th60:
  |.z1 - z2.| = |.z2 - z1.|
proof
  thus |.z1 - z2.| = |.-(z2 - z1).| .= |.z2 - z1.| by Lm26;
end;
