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:In all previous attempts [15, 30, 34, 8, 64, 43], the overlap problem is handled by partitioning the placement area and adding new constraints to the formulation to restrict the movement of the cells before solving another optimization problem.
Category: