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:1 The stratified bisimulation relations (Milner, 1980; Hennessy and Milner, 1985) ∼k⊆ Pr× Pr for k ∈ N are defined as follows:
Category: