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:Definition 0.2.1 The stratified bisimulation relations (Milner, 1980; Hennessy and Milner, 1985) ∼k⊆ Pr× Pr for k ∈ N are defined as follows:
• E ∼0 F for all E,F ∈ Pr • E ∼k+1 F iff for each a ∈ Act: if E
a−→ E′ then there is F ′ ∈ Pr such that F a−→ F ′ and E′ ∼k F ′; and if F a−→ F ′ then there…
Category: