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_3 -> u. u -> r_3. p <-> r_3.
r_3 <-> u can be inferred via the biconditional introduction rule. u <-> r_3 can be inferred via the biconditional introduction rule. p -> r_3 can be inferred via the biconditional elimination rule. r_3 -> p can be inferred via the biconditional elimination rule.