Hj-1 - {Py._uP2, P\,Ę > Ą-2 > Ą- 11
Pk+. is the next vertex exterior to H(1 ą+j.ą and ą+JĄ_2 aretangenttoHt l Hjr+j = (Ą+; > Ą > Ą-2 > Ę+j )