Typed Constituents: the FSET Construct

The fset special attribute expresses a completeness constraint in an FD.

          


;; Value of FSET is a list of symbols.
((a  ((fset (x y z))
      (x 1))))


          
          

FSET specifies that all symbols which are NOT listed is its value have a value of NONE, that is, they are not defined at this level of the fd.

          


;; b is not in the fset of a
> (u '((a ((fset (x y z))))) '((a ((b 1)))))
:fail


          
          

Two FSET descriptions can be unified. The result is an FSET whose value is the intersection of the two values.

          


> (u '((fset (x y z))) '((fset (v w x z))))
((fset (x z)))


;; ((fset nil)) is equivalent to NONE (no feature accepted)
> (u '((a ((fset (x y))))) '((a ((fset (a b))))))
((a none))