In section 3.1 we insisted that a polygon should have no three successive vertices collinear.
However, other collinearities are allowed. In particular, theorem 3.51 (pappus's theorem) may be rephrased as follows:
If each set of three alternate vertices of a hexagon is a set of three collinear points, and the three pairs of opposite sides intersect, then the three points of intersection are collinear.