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:…et al, 2009), parallel-machine scheduling problems (Lin et al, 2011), flowshop scheduling problems (Ribas et al, 2011; Pan and Ruiz, 2014), HFSPs (Ying, 2009; Ying et al, 2014), distributed flowshop scheduling problems (Lin et al, 2013b; Fernandez-Viagas and Framinan, 2015; Ribas et al, 2017),…
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