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: u -> r. r_8 -> u. r -> r_8. r_8 <-> q_6. ~q_6.
u -> r_8 can be inferred via the transitivity rule. r_8 -> r can be inferred via the transitivity rule. r -> u can be inferred via the transitivity rule. r_8 -> q_6 can be inferred via the biconditional elimination rule. q_6 -> r_8 can be inferred via the biconditional elimination rule. ~r_8 can be inferred via the biconditional elimination rule.