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:A materially stable constitutive model for the simulation of arterial walls has been developed in Holzapfel et al. (2000) (with extensions to the inelastic domain Gasser and Holzapfel, 2002; Holzapfel et al., 2002), where each layer of the artery is modeled as a fiber–reinforced material.
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