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:Although it is still not clear if an increased parasympathetic tone is due to the inflammatory state present in asthma or represents a pathophysiological mechanism by itself, the rationale for the application of cholinergic antagonists in asthma appears robust [16, 20] and the non-neuronal cholinergic represents a favorable target.
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