In references associated with books, the reference fields for publication data such as publisher, place, and edition are all taken from the data on the book-document itself. This unary relation captures these constraints in one place, so that each of the book reference types can just inherit them.
(<=> (Book-Publication-Data-Constraint ?Ref)
(And (Publication-Reference ?Ref)
(Value-Cardinality ?Ref Ref.Document 1)
(Value-Type ?Ref Ref.Document Book)
(Value-Cardinality ?Ref Ref.Publisher 1)
(Inherits-Publisher-From-Document ?Ref)
(Value-Cardinality ?Ref Ref.Year 1)
(Inherits-Year-From-Document ?Ref)))