Additional Exercise for Chapter 8 
 Recall the vehicle hierarchy from Section 5.4.2 in which we had a
 vehicle, Darling, that could be both a sub and a car. 
 In this problem, instead of simply belonging to two categories at
 once, the vehicle morphs from one mode into another; hence, the
 properties inherited by Darling are fluents.
 
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 Exercise:
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 Consider an action "morph" turning an object from a car to a sub, etc. 
 For simplicity, assume that an object can only belong to one class
 at a time and that morphing only occurs between leaf classes.
 Describe the following vehicle hierarchy and action morph in ASP.

                    vehicle
               /        |          \
     land_vehicle  sea_vehicle  air_vehicle
          |             |             |
         car           sub          plane
 
 Initially, Darling is a car and the Narwhal is a sub.