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_4 <-> u_4. u_4 <-> r_4.
r_4 -> u_4 can be inferred via the biconditional elimination rule. u_4 -> r_4 can be inferred via the biconditional elimination rule. u_4 -> r_4 can be inferred via the biconditional elimination rule. r_4 -> u_4 can be inferred via the biconditional elimination rule.