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_11 -> u_4. u_4 -> r_11. r_11 -> s_4. u <-> p_11. ~s_4.
~r_11 can be inferred via the modus tollens rule. p_11 -> r_11 can be inferred via the transitivity rule. u_4 -> s_4 can be inferred via the transitivity rule. u -> p_11 can be inferred via the biconditional elimination rule. p_11 -> u can be inferred via the biconditional elimination rule.