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Creating a Case-Base using CASL
1. Introduction
CASL is a language used for Case-Based Reasoning. The contents of a case-base are described in a file known as a case file, using the language CASL. The program Caspian uses this case file to create a case-base in the computer's memory, which can then be accessed and modified in order to solve problems, give a diagnosis etc. using Case-Based Reasoning techniques. When new cases are added to the case-base in the computer's memory, they are also appended to the end of the case file.

The process of Case-Based Reasoning (CBR) is to find a case that is similar to the current situation, modify the solution to fit the current situation and then to store the case in the case-base.
These processes can be carried out using the program Caspian. The purpose of this section is to describe the language used in the case file for specifying cases (CASL), and to describe how Caspian uses this information to create the case-base. You create a case base in CASL using your favourite editor and then load it into Caspian. Caspian checks that it is a legal CASL program as it loads it in (see the Caspian document for more details of error messages).

2. General Structure of a Case
In CASL, a case is similar to a record in a database. The basic unit is a field which may contain a string, a number, an enumeration symbol or a list. A list item may be any of the four basic types.

There are a number of differences between a CASL case and an ordinary database record: 1) There are two groups of fields in a case. The first group describes the situation and the nature of the problem. The second group describes the solution to the problem. 2) It is possible for a field to be omitted. This is only true for those fields which are not used for indexing (see below). Both field lists must contain at least one field. The problem section must use at least one field as an index.

3) Each case has a value known as the result. This can be the value SUCCESS, or the value FAILURE, or a value between -5 and 5.
4) A case has a name associated with it.
Enumeration fields defined in the problem section of a case may be used as indexes. General Syntax of the case file
The case file consists of a number of blocks of code. The overall syntax is: Introduction
Case Definition
Index Definition
[Modification Definition]
[Repair Rule Definition]
Case Instance {Case Instance}

CASL description document v1.3


© UW Aberystwyth, February 1995

The Introduction block contains introductory text which gets displayed when the program Caspian has finished checking the case file.
The Case Definition defines the types and the weights of the fields that may appear in a case. The information in the Case Definition is used for typechecking input cases while the weights are used to aid the case-matching process (described below). The Index Definition defines the fields used as indexes when searching for a matching case. A case base should have at least one field used as an index. The type of an index field must be an enumerated type.

The Modification definition defines the modification rules. The purpose of the modification definition is twofold:
1) It provides a means of specifying that certain symbols or numbers are similar, for matching purposes.
2) It provides a means of specifying symbols as abstractions of others. This is useful for making the search more general or for defining generalised cases. The Repair Rule definition contains the repair rules. The repair rules are used to modify the solution retrieved from the case-base, to make it more suitable for the current situation. Both the Modification definition and the Repair rule definition may be omitted, but a completed system should contain both, if the system is to be a true CBR system. The last set of blocks are the case instances. These are the cases that make up the case base. The case file must contain at least one case instance and will need to be...
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