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  L15 Selectional restrictions


       Selectional Restrictions & Semantic Attributes

o  Selectional restrictions are semantic constraints placed on 
   arguments of a verb.
    + Ex.:  The fork ate the carrot.
       - Violates semantic constraint:
            Only animate beings can eat.
       - Fork is not animate.

o  Example constraint
    + Eat
         ROLE     SEMANTIC FEATURE
       * agent      +animate
       * theme      +edible

    - Meaning:
       * Object which can be filler of agent role must be animate.
       * Filler of theme must be edible.

o  Requires putting semantic features with every noun.  Ex. features:
   living being, edible,  etc.
    - Nouns: 30000 upwards for reasonable system
      (Contrast with number of verbs: 3000)
    * To put semantic features with nouns is a large task

o  Selectional restrictions help in disambiguation
    * Identifying roles
       + Ex.1:  He opened the lock with the key.
       + Ex.2:  He opened the lock with the yellow color.
       - In Ex.1, key is instrument, but yellow color is not.

       *  Selecting word sense
           + Ex.1:  He drinks milk
           + Ex.2:  My car drinks petrol
           - Sense of 'drink' in Ex.2 is excessive consumption.
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