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 Zeilberger (1990), Chyzak (2000) and Koutschan (2013) for the holonomic approach, to Wilf and Zeilberger (1992), Wegschaider (1997) and Apagodu and Zeilberger (2006) for the multi-sum approach, or to Schneider (2005a, 2008, 2010c, 2015) for further refinements of the difference field approach.
Category: