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_2 -> q_19. ~r_2. ~q_19 -> q_3. q_19 -> r_2.
~q_19 can be inferred via the modus tollens rule. r_2 <-> q_19 can be inferred via the biconditional introduction rule. q_19 <-> r_2 can be inferred via the biconditional introduction rule.