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|Grammer 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.
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.