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A comparison of BIBFRAME and RDA in RDF (referred to below as RDA), in an 
attempt to map RDA to BIBFRAME, raised the issue of constrained vs 
unconstrained schemas.

The full set of RDA properties is constrained by the RDA classes of Agent, 
Work, Expression, Manifestation and Item.  That is, each property is related 
to a specific class when appropriate:  e.g. abridgementOfExpression and 
abridgementOfWork.  A parallel set of properties has been created where the 
constraints of class are lifted:  e.g. abridgementOf.  This unconstrained 
version of RDA loses the context of some properties but is intended to 
facilitate mapping to schemas that do not use the FRBR model underlying RDA.

BIBFRAME is a constrained schema, but constrained by different classes: 
Agent, Work, and Instance.  There is no unconstrained version of BIBFRAME.

A mapping of RDA to BIBFRAME presents choices and challenges.

Is it better to use constrained RDA, which causes explicit conflicts of 
domain:  e.g. mapping rdam:reproductionOfManifestation to bf:reproduction 
and rdai:reproductionOfItem to bf:reproduction?

Or is it better to use unconstrained RDA, which still has conflicts (an 
unconstrained domain vs a constrained one in BIBFRAME):  e.g. mapping 
rdau:reproductionOf to bf:reproduction?

It is not obvious which is the better choice.  Although perhaps we need both 
mappings, each with its own problems regarding original and destination 
domains.

A corollary of the question is that any roundtrip RDA -> BF -> RDA is lossy. 
If constrained RDA is used as a starting point, RDA classes are lost in the 
mapping itself, and if unconstrained RDA is used, classes are lost prior to 
mapping.  Either way, RDA classes cannot be recovered in a BF -> constrained 
RDA mapping.