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 L03 FS: Feature-structures


             Feature  Structures

o  Feature structures (FS)
   *  FS store information about some entity (typically,
      linguistic entity such as a word, a phrase, etc.). 
       - Information expressed by means of attribute-value pairs

   -  FS is set of attribute-value pairs
       * Written in matrix-like notation

          [ CAT        n]
          [ NUMBER    sg]
          [ PERSON     3]

   -  An attribute is atomic
       + Ex. CAT, NUMBER, PERSON

   -  Values can be atomic
      +  Ex. n, sg, 3
      -  A FS with only atomic values, called simple FS

   -  A value can be a FS (in which case the FS is
      called complex or nested FS)

      +  Ex:  Nested feature structure (or complex FS)
          -  Separate CAT and agreement features
      
               [ CAT         n            ]
               [ AGRMNT      [NUMBER  sg] ]
               [             [PERSON   3] ]
      
          -  Value of feature AGRMNT is a feature structure.

o  Feature path
     (AGRMNT  NUMBER)

o  Graph notation for feature structure.

        CAT /----->------o n
           /
        ->o                  NUMBER
           \  AGRMNT      /---->---o sg
            \------>-----o
                          \---->---o 3 
                            PERSON
   - The arc labels are attributes
   - The node is a value, that is:
     * Leaf node is an atomic value
     * Non-leaf node is a FS
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