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_6 -> u_2. u <-> u_2. p -> r. p <-> p_6. ~u. r -> p.
p <-> r can be inferred via the biconditional introduction rule. r <-> p can be inferred via the biconditional introduction rule. u -> u_2 can be inferred via the biconditional elimination rule. p -> p_6 can be inferred via the biconditional elimination rule. u_2 -> u can be inferred via the biconditional elimination rule. p_6 -> p can be inferred via the biconditional elimination rule. ~u_2 can be inferred via the biconditional elimination rule.