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. r_1 <-> q_4. r_1 -> t_1.
~r_1 can be inferred via the modus tollens rule. r_1 -> q_4 can be inferred via the biconditional elimination rule. q_4 -> r_1 can be inferred via the biconditional elimination rule.