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:Other alternatives include an extension of the well-known (μ + λ) evolution strategy (ES) for mixed integer problems, MIES (Li et al., 2013) and the covariance matrix adaptation evolutionary strategy CMA-ES (Hansen, 2011).
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