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: w <-> u. w -> r_1. u <-> r_1.
w -> u can be inferred via the biconditional elimination rule. u -> r_1 can be inferred via the biconditional elimination rule. u -> w can be inferred via the biconditional elimination rule. r_1 -> u can be inferred via the biconditional elimination rule.