
theorem Th15:
  for x, y being Complex st x <> y or x - y <> 0 holds Arg(
  x-y) < PI iff Arg(y-x) >= PI
proof
  let z1,z2 be Complex;
  assume z1<>z2 or z1 - z2 <> 0;
  then z1-z2<>0c;
  then Arg(z1-z2)<PI iff Arg(-(z1-z2))>=PI by Th14;
  hence thesis;
end;
