We thank the anonymous reviewers and Giorgio Satta for their valuable comments.
let G = (T, N, S, R) be a Context-Free Grammar in Chomsky normal form with no useless productions, where T is a finite set of terminal symbols, N is a finite set of nonterminal symbols (disjoint from T ), S ∈ N is a distinguished nonterminal called the start symbol, and R is a finite set of productions of the form A → B C or A → w, where A, B, C ∈ N and w ∈ T .