Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Indeed, the non-monotonic reasoning community has continuosly produced, in the past, several extensions of nonmonotonic logic languages, aimed at improving readability and easy programming through the introduction of new constructs, employed in order to specify classes of constraints, search spaces, data structures, new forms of reasoning, new special predicates [2, 9, 18], such as aggregate predicates [4].
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