**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.