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: r <-> w_1. u_3 -> r. r -> u_3.
u_3 <-> r can be inferred via the biconditional introduction rule. r <-> u_3 can be inferred via the biconditional introduction rule. r -> w_1 can be inferred via the biconditional elimination rule. w_1 -> r can be inferred via the biconditional elimination rule.