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:For the formal definition of safety an areaS was introduced in (Rüdiger et al., 2007a) and further discussed in (Rüdiger et al., 2007b), which leads to catastrophic consequences when left.
Category: