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: p -> u. r_4 <-> u. r_4 -> p.
r_4 -> u can be inferred via the transitivity rule. r_4 -> u can be inferred via the biconditional elimination rule. u -> r_4 can be inferred via the biconditional elimination rule.