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:More recently, engineering lignin structures to facilitate the recovery and conversion of lignin into many types of aromatic commodities has also received considerable research attention (Ragauskas et al. 2014; Beckham et al. 2016; Rinaldi et al. 2016).
Category: