“…Then either g splits f , in which case, by Lemma 33, By definition, if Σ splits S, then there are sweep-outs f and g representing S and Σ, respectively, such that f × g is generic and g splits f . As noted above, the boundaries of the closures of R a and R b are edges of the graphic for f × g. As pointed out in [8], a horizontal tangency in the graphic for f × g corresponds to a critical point in the function g. Since g is a sweep-out, it has no critical points away from its spines, and so there can be no horizontal tangencies in the interior of the graphic. Thus the maxima of the upper boundary ofR a and minima of the lower boundary ofR b are vertices of the graphic.…”