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:…inactivation (Charlton et al., 1982; Chaudhuri et al., 2005; Forsythe et al., 1998; Xu and Wu, 2005) and vesicle depletion (Custer et al., 2006; Pieribone et al., 1995; Schneggenburger et al., 2002; Xu and Wu, 2005) have varying dependence on the frequency of stimulation at different synapses,…
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