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_2 -> s_4. s_4 -> p_2. s_4.
p_2 can be inferred via the modus ponens rule. p_2 <-> s_4 can be inferred via the biconditional introduction rule. s_4 <-> p_2 can be inferred via the biconditional introduction rule.