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:Dense Trajectory features are formed by the sequence of displacement vectors in an optical flow field, together with the HoG-HoF descriptor (Laptev et al, 2008) and the motion boundary histogram (MBH) descriptor (Dalal et al, 2006) computed over a local neighbourhood along the trajectory.
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