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:Let us note that co-occurrence is nowadays a common method to find a relationship between biomedical concepts; co-occurrence methods are commonly used to discover new and hidden relations, following the seminal work of Swanson (see [15], [25] and the more detailed works [26–30]).
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