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 -> s_1. s_1 <-> p_13. p_13 -> u. ~u. s_1 <-> p.
~p_13 can be inferred via the modus tollens rule. s_1 -> p_13 can be inferred via the biconditional elimination rule. s_1 -> p can be inferred via the biconditional elimination rule. p_13 -> s_1 can be inferred via the biconditional elimination rule. p -> s_1 can be inferred via the biconditional elimination rule.