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Suppose, for example, that you want to define isVegetable, a
predicate that determines whether a Food instance is also instance
of the Vegetable class. First, you capture the fact that most
Food instances are not instances of the Vegetable class by
defining isVegetable for the Food class:
Food method definition • instance isVegetable ^ false
Next, you capture the fact that Vegetable instances are instances of
the Vegetable class:
Vegetable method definition • instance isVegetable ^ true