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 first method used the CADP (bisimulation) equivalence checker to compare the models MI that we learned for the mutant implementations I with the model MR learned for the reference implementation R.2 For each of the mutants, CADP quickly found a counterexample trace illustrating the difference between the models of the mutant and the model of the reference implementation (for each mutant it takes around 3 seconds to find the counterexample).
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