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_9 or ~u_4. t <-> q_9.
u_4 -> q_9 can be inferred via the rewriting rule. t -> q_9 can be inferred via the biconditional elimination rule. q_9 -> t can be inferred via the biconditional elimination rule.