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