Furthermore, if S i and S j have maximum intersection λ, then any subset of S i also has maximum intersection λ with any subset of S k , and we need not specify those constraints either.
now, let a choke-vertex in the partial order X, be an element u ∈ X such that for every v, w ∈ X where v is a successor of w and u v, we have u w. any chain of successors from elements not after u to elements after u, must pass through u.