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:[4] have recently proposed a scheme for an entanglement amplifier based on a nondegenerate three-level laser when the three-level atoms are injected at the lower level and the top and bottom levels are coupled by a strong coherent light.
Category: