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_3. r_4 -> q_6. q_6 -> r_4.
q_6 -> u_3 can be inferred via the transitivity rule. r_4 <-> q_6 can be inferred via the biconditional introduction rule. q_6 <-> r_4 can be inferred via the biconditional introduction rule.