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:To model a large class of such situations von Neumann and Morgenstern (1944) came up with a definition of a simple game which is the simplest possible type of a cooperative game, where the payoffs of coalitions are either 1 or 0, i.
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