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: t_1 <-> q_21. t_1. forall x_9: P_9(x_9).
t_1 -> q_21 can be inferred via the biconditional elimination rule. q_21 -> t_1 can be inferred via the biconditional elimination rule.