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:Plan and goal recognition models have been devised for story understanding systems (Charniak and Goldman, 1993), productivity software (Horvitz, et al., 1998), intelligent tutoring systems (Conati, Gertner, and VanLehn, 2002), and dialogue systems (Blaylock and Allen, 2003).
Category: