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: q_18 -> q. r -> q_9. s <-> q_9. s <-> q_18.
s -> q_9 can be inferred via the biconditional elimination rule. s -> q_18 can be inferred via the biconditional elimination rule. q_9 -> s can be inferred via the biconditional elimination rule. q_18 -> s can be inferred via the biconditional elimination rule.