The hierarchical structure of scenarios

   A scenario is not a single, monolithic structure. Rather, a scenario is made up of smaller pieces, forming a hierarchical knowledge structure. At the lowest level, a series of actions and states are known to exist. A temporal order is assumed to exist, although every time step may not be represented. Similarly, although causal relations exist between the known states and actions, every action may not be related to the actions around it.

This can be seen in Squire. At the lowest level, the scenario proceeds as follows, in terms of the actions:

trap $\Rightarrow$ confront $\Rightarrow$ reveal $\Rightarrow$ rescue

  A set of co-temporal states are referred to as a scene, a sort of mega-state describing several elements at once. This will describe where the action is taking place, who is going to be involved, and so forth. Chains of actions and states which occur in the same scene are said to be   events. So, at the next level of hierarchical description, a scenario consists of scenes and events. The trap and confront actions in the story are elements of the same event. They are directly related with the intervening state of being captured; there is also an implied temporal ordering to these actions. The story begins with the scene describing the initial situation in which the Enterprise is in. The reveal action is an element of another event; finally, the rescue makes up the last event.

  The highest level at which a scenario can be described is the segment. A segment is a set of related events which may cross over several scenes. A segment bears a close resemblance         to the older idea of scripts and plans ([#!scripts:schank-abelson-1977!#]). In a story as short as Squire, a single segment is sufficient--in this case, the capture-segment serves to illustrates the story. This segment describes the actions which make up a typical capture situation--an agent gains control of something or someone else through one of a number of possible plans; the something wants to get away and may try a number of plans; and one possible resolution is the freeing of the captured object. In a longer story, such as Lycanthrope, more segments are needed to capture the actions of the tale. At the highest level, then, a scenario consists of a set of segments. These can be further related by being described as co-occurring, occurring in an interleaving fashion, occurring in a temporally ordered manner, and so forth.