So far, we have seen restrictions that define classes based on the presence of certain values for given properties. OWL allows a much finer-grained way to define classes, based on the number of distinct values a property takes. Such a restriction is called a cardinality restriction. This seemingly simple idea turns out to have surprising subtlety when modeling in OWL. Cardinality restrictions allow us to express constraints on the number of individuals that can be related to a member of the restriction class. For example, a baseball team has exactly nine (distinct) players. A person has two (biological) parents. Cardinality restrictions can be used to define sets of particular interest, like the set of one-act plays or the set of ...

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