, H n+1 , where H j is the subset of H that depend on X j but do not depend on any variables ordered before X j , and H n+1 is the subset of H that do not depend on any variables at all (i.e., they are constants).
3 H j = {H ∈ H|X j ∈ d(H), ∀i < j X i ∈ d(H)} and H n+1 = {H ∈ H|d(H) = ∅}.