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|Grammar rule attribute_goals//1|
The latter property ensures that users can reason about residual goals, and see for themselves whether a constraint library behaves correctly. It is this property that makes it possible to thoroughly test constraint solvers by contrasting obtained residual goals with expected answers.
This nonterminal is used by copy_term/3, on which the Prolog top level relies to ensure the basic invariant of pure Prolog programs: The answer is declaratively equivalent to the query.
The copy_term/3 primitive uses attribute_goals//1 inside a findall/3 call. This implies that attribute_goals//1 can unify variables and modify attributes, for example, to tell other hooks that some attribute has already been taken care of. This nonterminal is also used by frozen/2 which does not create a copy. Ideally attribute_goals//1 should not modify anything to allow direct application in frozen/2. In the current implementation frozen/2 backtracks over attribute_goals//1 to tolerate the current behavior. This work-around harms the performance of frozen/2. New implementations of attribute_goals//1 should avoid relying on backtracking when feasible. Future versions of frozen/2 and copy_term/3 may require attribute_goals//1 not to modify any variables or attributes.
Note that instead of defaulty representations, a Prolog list is used to represent residual goals. This simplifies processing and reasoning about residual goals throughout all programs that need this functionality.