However, the binarization of Figure 4 allows the factorization into S(U , NP ) ↔ S(U , NP ) and U : @(NP , V ) ↔ @(V , NP ), which are fully binarized productions.
in general, STSGs (or SCFGs or extended tree transducers) cannot be fully binarized as shown in Aho and Ullman (1972).