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:The literature (Sugihara and Smith, 1997; Shahidi et al., 2004; Gerke, 1999; Gao and Tian, 2007; Zhao and Yan, 2005) shows most of the non-traditional algorithms are applied only for regular convex polygonal obstacles, not for irregular convex, concave polygonal or curved obstacles.
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