A PCFG G consists of: • A finite set of nonterminal symbols N; • A finite set of terminal symbols Σ; • For each A ∈ N, a set of rewrite rules R(A) of the form A → α, where α ∈ (N ∪ Σ) * , and R = ∪ A∈N R(A); • For each rule A → α, a probability θ A→α .
the collection of probabilities is denoted θ, and they are constrained such that: θ is grouped into |N| multinomial distributions.