The Special Attribute CAT: General Outline of a Grammar

Each constituent of an FD is generally characterized by its ``category''. In FD terms, this means each constituent includes a feature of the form (CAT category-name), where category-name is expected to be an atom.

A grammar is expected to give directives for each possible category, for example NP, VP, or NOUN. The outline of a grammar must be:

          
((alt (
       ((cat s)
        <rest of grammar for category S>)
       ((cat np)
        <rest of grammar for category NP>)
       <other categories>
      )))
          
          

NOTE: The current version of the unifier makes the assumption that the grammar has such a form. The (CAT xxx) pairs must appear first. The function grammar-p checks that a grammar has the right form. The list of categories known by the grammar can be found by using the function list-cats. See appendix for a list of the non-standard features of this implementation.    

NOTE: The symbol identifying categories ('cat) can be changed in the program. It is by default 'cat, but this default can be changed by setting a new value to the *cat-attribute* variable or by providing an optional argument to the unification functions, as explained in the reference manual.