theorem
  for p1,p2 st Arg(p1)<PI & Arg(p2)<PI holds Arg(p1+p2)<PI
proof
  let p1,p2;
  assume Arg(p1)<PI & Arg(p2)<PI;
  then Arg(euc2cpx(p1)+euc2cpx(p2))<PI by COMPLEX2:20;
  hence thesis by Th9;
end;
