theorem
  for p1,p2,p3 st angle(p3,p2,p1) <>0 holds angle(p3,p2,p1)=2*PI-angle(
  p1,p2,p3)
proof
  let p1,p2,p3;
  assume angle(p3,p2,p1) <>0;
  then angle(p3,p2,p1)+angle(p1,p2,p3) = 2*PI by COMPLEX2:80;
  hence thesis;
end;
