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Hooks in Query Processing

As explained above, the major parts of query processing are implemented in the 'SU_messages' module in the file library('SU_messages') through the following extendible predicates:

This is to enable the user to change the language used, the processing done, etc., simply by changing or replacing the library('SU_messages') file.

To give more control to the user and to make the system more robust (for example if the 'SU_messages' module is corrupt) the so-called four step procedure is used in the above three cases--obtaining the query class parameters, performing the query input and performing the mapping. The four steps of this procedure, described below, are tried in the given order until the first one that succeeds. Note that if an exception is raised within the first three steps, then a warning is printed and the step is considered to have failed.