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:In their classic paper (Hennessy and Milner, 1985), Hennessy and Milner defined an alternative non-increasing (with respect to set inclusion) sequence 'i, i ≥ 0, of approximants to bisimilarity as follows:
• P '0 Q always holds.
Category: