example with which we began this paper, the choice of support for the picture frame depended upon the type of wall material out of which the wall area was constructed. Here we have an example of an interaction between object types, the support and the wall, where the wall already has a definite reference. In this planning protocol a chaining of default rule application was pursued to determine the type of wall material involved. We will return to this example shortly since it illustrates a computational role for dispositional expressions. What should be noted here is that the basis for each default assumption is annotated. Consequently, when chaining is allowed, these annotations provide a basis for controlling and evaluating the inference chain. For example, it provides the information needed to detect a cycle in the inference chain.

14 An Abbreviated and Stylized Trace

A number of pieces of commonsense knowledge have been discussed and cast in a particular computational form. What remains to be done is to provide a recapitulation of this discussion which shows how these pieces fit together to form the puzzle of commonsense planning posed in this paper. What follows then is an abbreviated sketch of the computational moves that could yield the planning protocol with which the paper began.

     Figure 2 provides a hopefully readable approximation to the major steps that might underlie the planning protocol. To make this trace easier to follow, I have avoided a strict encoding of the trace into the language of Descforms. Most notably, the Alist notation has been dropped in favor of a notation that uses the name of the binding within the Desc itself. In order to indicate in a rough way the degree of definiteness of an expression, the following conventions are followed in this figure. A constant which is bound to a unique WM object appears in large capitals, one that is not uniquely bound is shown in small capitals, and a constant that is currently unbound to a WM constant is rendered in italics. Relations are shown in small letters. The Preconditions of an action are preceded by the name of the action concatenated with a ''P" and a number so that they may be simply referred to at other points in the trace.

     The trace begins by using the goal as the Query-Descform against the Goal-Select rules. Since the goal states that a frame and wall are involved, the appropriate sense of HANG is retrieved. This sense of HANG adds the new information that a support must be involved in the action. This support for the frame is unbound to any WM object.

     This action Desc now serves as the Query-Descform for accessing the appropriate Precondition rules. Three preconditions are retrieved. The first is rather uninteresting. The second tells us that the support for the frame should be embedded in the center of the wall area where the picture is to be hung. The third introduces three new types of objects; picturewire and supports on the frame for the wire.

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