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_1 -> q_6. ~r_1. r_4 -> q_6. q_6 -> r_1.
~q_6 can be inferred via the modus tollens rule. r_4 -> r_1 can be inferred via the transitivity rule. r_1 <-> q_6 can be inferred via the biconditional introduction rule. q_6 <-> r_1 can be inferred via the biconditional introduction rule.