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: ~r_4 -> q. r_4 <-> u. ~u.
r_4 -> u can be inferred via the biconditional elimination rule. u -> r_4 can be inferred via the biconditional elimination rule. ~r_4 can be inferred via the biconditional elimination rule.