However, such a derivation is ruled out automatically by the mechanisms of a TAG derivation and feature unification. Suppose it was possible to first compose the want tree with the dance tree and then insert the persuade tree. (This is not possible in the XTAG grammar because of the convention that auxiliary trees have NA (Null Adjunction) constraints on their foot nodes.) Even then, at the end of the derivation the control feature of the subject of want would end up co-indexed with the PRO subject of persuade and the control feature of Mary would be co-indexed with the PRO subject of dance as desired. There is no way to generate the illegal co-indexing in ((63)). Thus the locality constraints on PRO control fall out from the mechanics of TAG derivation and feature unification.