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:Recently, peptide fragmentation with tandem mass spectrometry parallel to the standardized MALDI-TOF-MS has emerged as an additional tool which provides enhanced capabilities for peptide sequencing directly from samples such as nervous tissues or even single cells of invertebrates, including those of insects (e.g., Yew et al., 2003; Billimoria et al., 2005; Neupert and Predel, 2005; Nachman et al., 2006; Wegener et al., 2006).
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