An FD (functional description) is a data structure representing constraints on an object. It is best viewed as a list of pairs (attribute value). Here is a simple example:
((article `the') (noun `cat'))
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There is a function called fd-p in the package that lets you know whether a given Lisp expression is a valid FD or not and gives you helpful error messages if it is not. In FUGs, the same formalism is used for representing both the input expressions and the grammar.