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 <-> u_4. ~u_4. r_4 <-> t. r_4 <-> u.
u -> u_4 can be inferred via the biconditional elimination rule. r_4 -> t can be inferred via the biconditional elimination rule. r_4 -> u can be inferred via the biconditional elimination rule. u_4 -> u can be inferred via the biconditional elimination rule. t -> r_4 can be inferred via the biconditional elimination rule. u -> r_4 can be inferred via the biconditional elimination rule. ~u can be inferred via the biconditional elimination rule.