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:There are two main streams of works and methods that can be listed: 1) exact algorithms on tree graphs [28, 29, 24, 21, 12], 2) ad hoc heuristics approaches without any guarantee for general graphs [27, 24, 23].
Category: