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A declaration
:- dynamic :PredSpec, …, :PredSpec. ISO
where each PredSpec is a predicate specification, causes the specified predicates to become dynamic, which means that other predicates may inspect and modify them, adding or deleting individual clauses. Dynamic predicates are always stored in interpreted form even if a compilation is in progress. This declaration is meaningful even if the file contains no clauses for a specified predicate—the effect is then to define a dynamic predicate with no clauses.
The semantics of dynamic code is described in ref-mdb-bas. See mpg-ref-dynamic.