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 <-> q_9. r_4 <-> q.
r_4 -> q_9 can be inferred via the biconditional elimination rule. r_4 -> q can be inferred via the biconditional elimination rule. q_9 -> r_4 can be inferred via the biconditional elimination rule. q -> r_4 can be inferred via the biconditional elimination rule.