In response to this difficulty, researchers striving to build usable systems have experimented with a variety of strategies. One is to pre-equip the dialogue system with a hierarchy of carefully engineered plans suitable for the type of dialogue to be handled (such as tutoring, repair, travel planning or schedule maintenance), and to choose the logical vocabulary employed in NLU/NLG so that it meshes smoothly with both the planning operators and with the surface realization schemas aimed at the target domain. (As a noteworthy example of this genre, see Moore &amp; Paris 1993.) In this way domain and text planning and surface realization become relatively straightforward, at least in comparison with systems that attempt to synthesize plans from scratch, or to reason extensively about the world, the interlocutor, the context, and the best way to express an idea at the surface level. But while such an approach is entirely defensible for an experimental system intended to illustrate the role of plans and intentions in a specialized domain, it leaves open the question of how large amounts of linguistic knowledge and world knowledge could be incorporated into a dialogue system, and used inferentially in planning communicative (and other) actions.