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 -> ~p_1. s_4 -> t. p_1. t -> s_4.
~t can be inferred via the modus tollens rule. s_4 -> ~p_1 can be inferred via the transitivity rule. s_4 <-> t can be inferred via the biconditional introduction rule. t <-> s_4 can be inferred via the biconditional introduction rule.