V = { π | ∃B. [ B ] π∊blocks(S) }
as we have already defined the set of labels of a program via the function labels. So I think set of vertices could be equivalently and more simple defined as:
V = labels(S)
. Maybe you want to change that for the next semester? Or am I wrong?

EDIT: Similarily on page 25 we again write { π | ∃B. [ B ] π∊blocks(S) } although we could simply use label(S). Right?