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Semantic Web for the Working Ontologist, 2nd Edition by James Hendler, Dean Allemang

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Unions and Intersections

We begin with the basic logical combinations, which are familiar from set theory. OWL provides a facility for defining new classes as unions and intersections of previously defined classes. All set operations can be used on any class definition at all in OWL, including named classes and restrictions. This allows OWL to express a wide variety of combinations of classes and conditions. The semantics for the constructors as one would expect, matching the set operations of the same name.

Syntactically, they use the list constructs of RDF, as follows:

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The union of two or more classes includes the members of all those classes ...

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