for each argument A ∈ V τ , we have a rule equivalence class We can see that in grammar G ′ each N ∈ B eventually ends up rewriting to one of N 's expansions β in G. There are two indirect paths, one through N b 1 and one through N b 2 .
this defines the probability of N → β in G, θ N →β , as the probability of rewriting N as β in G ′ via N b 1 and N b 2 .