What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: u_1 -> r_8. r_8 -> u_1. p_8 -> w. w -> r_8.
p_8 -> r_8 can be inferred via the transitivity rule. w -> u_1 can be inferred via the transitivity rule. u_1 <-> r_8 can be inferred via the biconditional introduction rule. r_8 <-> u_1 can be inferred via the biconditional introduction rule.