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:This does not a¡ect the probability calculations for any reversible Markov-process model of nucleotide substitution (Felsenstein 1981), and de¢nes a direction (away from the root) on each branch of the tree.
Category: