Exercise 3. Four rays OA, OB, OC, OD issue from O (in the order listed)
such that AOB = COD and BOC = DOA. Show that OA and OC are collinear,
as are OB and OD.
Exercise 4. If four consecutive rays OA, OB, OC, OD are such that the
bisectors of angles AOB, COD are collinear, as are the bisectors of BOC, AOD,
then these rays are collinear in pairs.