theorem
  z1 - -z2 = z1 + z2
proof
  reconsider z19=z1,z29=z2 as Element of COMPLEX by Def1;
  -z2 = -z29 by Th2;
  hence z1 - -z2 = z19- -z29 by Th3
    .= z1 + z2;
end;
