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:GLS Feeding
p-OHB GLS was extracted from seeds of white mustard (Sinapis alba) (SeedCom) as previously described (Thies, 1979; Zrybko et al., 1997).
Category: