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You could, of course, define isVegetable for the Object
class, rather than for the Food class. Generally, however, you
should define such a predicate for the lowest class in the class hierarchy
that is a superclass of all classes whose instances are possible receivers
of the predicate; otherwise, you clutter up the definition of the
Object class and confound your use of the file-out mechanism,
described in Chapter••missing reference••.