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:Previous work suggested that once Lprod, the length of the strand exchange product, exceeds ∼20 bp, bacteria are committed to strand exchange (19); however, studies of 21 bp sequences in different strains of Escherichia coli reveal that particular strains have from ∼10 000 to 175 000 unique sequences repeated at least once (20,21).
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