We discussed above why top-down planning seems an unnatural basis for formulating an NLG model that can exploit opportunities. There are, however, other models of planning that may be more appropriate.
ILEX is inspired loosely by ideas from opportunistic planning [Hayes-Roth and Hayes-Roth 79,Pryor 96]. Key elements of this are:
Pryor's work is implemented in PARETO, a planner for a simulated robot delivery vehicle. The vehicle is given orders to deliver various objects to various building sites, and needs to locate these objects at other sites. The system is opportunistic in that while the truck is working on one goal, it is always ready to switch to another if an object on its find-and-deliver list turns up. For example, if the truck stops at one place to pick up a hammer, it may notice a saw, which is also on its list, and thus pick it up and proceed to its delivery point.
Pryor's planning occurs within a limited horizon---the robot only has certain knowledge in regards to the immediate location, and outside of that, the world is uncertain (objects are sometimes randomly moved between sites in the world).
ILEX inhabits a world analogous in certain respects to PARETO's: each page is a site on the map, and it is up to us to find opportunities for realising our goals at each site. However, while in the truck world the system is in control of motion to the next site, in the museum, it is the user who chooses the next page. Conversely, while objects outside the truck's immediate vicinity may move autonomously, for ILEX, facts and their values do not change.
Opportunistic planning has similarities with a number of other approaches to planning. It shares with incremental planning (used in NLG by [Cawsey 92]) the idea of starting to execute a plan before the plan is complete, and being prepared to repair the partial plan in the light of feedback. It shares with reactive planning the idea of being directed as much by the characteristics of the state of the world at execution time as by the pursuit of preconceived goals. However, unlike pure reactive planning it does acknowledge the need for explicit plans to be manipulated and it differs from many models of incremental planning in the extent to which the original plan can be diverted to exploit the characteristics of the world at execution time.