Proposition 5.1: S is contained in the complement of the union of the baselines of F.
Pf: It follows from the definition of a flock, that the primary baselines can not intersect S. Suppose that a secondary baseline, l, intersected S at a point P. The plane determined by V and l contains the generator VP of the cone. This plane also contains a line of intersection of two planes in the flock. Since these two lines must meet, there would be a point of the cone on the intersection of two flock planes. Thus, no secondary baseline can meet S, and so, S is contained in the union of the baselines of F.