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:Here, we report our analyses of fungal co-occurrences in seeds from our field study (Shipunov et al., 2008), and new inoculation experiments to test the three hypotheses that might explain, singly or in combination, the limited infection that has been observed: the environmental limitation…
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