theorem
  for p1,p2,p3 st angle(p1,p2,p3) =0 holds Arg(p1-p2)=Arg(p3-p2) & angle
  (p3,p2,p1)=0
proof
  let p1,p2,p3;
  assume
A1: angle(p1,p2,p3) =0;
  euc2cpx(p1)-euc2cpx(p2)=euc2cpx(p1-p2) & euc2cpx(p3)-euc2cpx(p2)=euc2cpx
  (p3- p2) by Th15;
  hence thesis by A1,COMPLEX2:74;
end;
