The modify step uses the result of the critique step to find an appropriate modification. Given a component that must be modified, finding the appropriate repair action is not immediately obvious. Like every step of our PCM-method the modification uses problem-type specific knowledge, such as the properties of components. For example, the repair-action of strengthening the Selection component results in checking for which possible Selection components this holds (for example: ``number-minimal'' is stronger then ``subset-minimal'')
Another example of knowledge that is useful for modifications of methods is whether configurations (methods) give the same solutions. This enables us to exclude modifications before verifying, and therefore to avoid the expensive simulation verify.
For example, in diagnosis we have the knowledge that when the computed sets of explanations are equal, we know that using the same values for the Selection and Solform components will result in the same solutions for these two methods. We can use this in avoiding a useless repair-action:
(here Es and Sols refer to the variables of the same name in formula (1)). A way to establish that same-Es holds is to use knowledge about properties of the components that are used.
A modification action can consist of modifying an individual component so that it has a desired property, modifying an entire method so that it has a desired property, or tuning components so that they become more compatible. We have mainly studied modification of methods. Finding the appropriate modification step can be a complex process that might consist of generating possible repairs, and preferring those that are ``closest'' to the original component. In Section 6.2, we illustrate such a complex repair-action.