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:The minimization of Kullback divergence does implement the above first point well, and thus this is why it has been used in the early stage of the BYY system (Xu, 1996, 1997, 1998b).
Category: