theorem
  for p1,p2,p3 holds angle(p1,p2,p3)=angle(p1-p2,0.TOP-REAL 2,p3-p2)
proof
  let p1,p2,p3;
  euc2cpx(p1)-euc2cpx(p2)=euc2cpx(p1-p2) & euc2cpx(p3)-euc2cpx(p2)=euc2cpx
  (p3- p2) by Th15;
  hence thesis by Th17,COMPLEX2:71;
end;
