Derived datasets can be defined implicitly or explicitly. An implicit
definition (of dataset O in terms of datasets I) is a logical
specification involving the source data I and the interface data O.
It is a valid definition of O in terms of I, if any two models of the
specification agreeing on I agree on O. In contrast, an explicit
definition is a query that produces O from I. Variants of Beth's
theorem state that one can convert implicit definitions to explicit ones.
Further, this conversion can be done effectively given a proof witnessing
implicit definability in a suitable proof system. We prove the analogous
effective implicit-to-explicit result for nested relations: implicit
definitions, given in the natural logic for nested relations, can be
effectively converted to explicit definitions in the nested relational calculus
NRC. As a consequence, we can effectively extract rewritings of NRC queries in
terms of NRC views, given a proof witnessing that the query is determined by
the views