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_4 <-> u_2. ~r. p_4 <-> r.
p_4 -> u_2 can be inferred via the biconditional elimination rule. p_4 -> r can be inferred via the biconditional elimination rule. u_2 -> p_4 can be inferred via the biconditional elimination rule. r -> p_4 can be inferred via the biconditional elimination rule. ~p_4 can be inferred via the biconditional elimination rule.