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:Bond graphs are a domain-independent notion of physical systems modeling, where the physical processes are directly represented as vertices in a directed graph (Breedveld, 1984; Thoma, 1989; Cellier 1991) and the edges represent the ideal exchange of energy between the vertices.
Category: