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: p_1 -> r_1. r_1 -> ~r. r_1 -> p_1. r.
~r_1 can be inferred via the modus tollens rule. p_1 -> ~r can be inferred via the transitivity rule. p_1 <-> r_1 can be inferred via the biconditional introduction rule. r_1 <-> p_1 can be inferred via the biconditional introduction rule.