Let us consider the comprehension process in the case study of the fire extinguisher. At this stage of processing, KA's working memory contains three elements: the interpretations of the sentences in the text generated by the language process, the SBF model of the spray can retrieved from the long-term memory, and the symbolically-represented structural model of the fire extinguisher given as part of the input to the system. The current task is to adapt the SBF model of the spray can to construct a model of the fire extinguisher.
The comprehension process notes the sentence interpretations in the working memory and the differences between these interpretations and the SBF model of the spray can. For example, it notes that the fire extinguisher and the spray can contain different substances under pressure. In addition, the comprehension process notes the structural differences between structural models of the fire extinguisher and the spray can. For example, it notes that the two contain different kinds of nozzles. It uses these differences to select generic model-modification plans that help to reduce specific differences and are indexed by the differences they can help to reduce. Examples of model-modification plans include the substance-substitution plan and the component-replacement plan. Given a specific difference between a component in the spray can and a component in the fire extinguisher, instantiating the latter plan in the context of the SBF model of spray can, for example, results in replacing each occurrence of the spray-can component in the SBF model by the corresponding component in the fire extinguisher. The application of this plan also results in the propagation of the causal effects of the new component. The structural models help to establish correspondences between the components in the spray can and the fire extinguisher. The SBF model of the spray can helps to focus the process of plan instantiation and application. The invocation and application of selected model-modification plans, one for each difference between the fire extinguisher and the spray can, results in the generation of a SBF model for the fire extinguisher. This preliminary SBF model provides a functional and causal explanation of the working of the fire extinguisher.
Next, KA evaluates the preliminary model of the fire extinguisher for both internal and external consistency. In reference to internal consistency, the comprehension process makes sure that no new element introduced into the SBF model is inconsistent with any other element. This is done by systematically tracing through the causal behaviors of the new SBF model. If an inconsistent element is detected, then the process retracts the corresponding modification from the SBF model. In reference to external consistency, the comprehension process makes sure that no element in the SBF model is inconsistent with the output of the language process. This is done by cross-checking of the SBF model and the sentence interpretations generated by the language process. Again, if an inconsistent element is detected, then the comprehension process retracts it from the SBF model. The final SBF model is KA's best explanation of the working of the fire extinguisher.