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_1 -> s. s <-> u_1. ~s.
~u_1 can be inferred via the modus tollens rule. s -> u_1 can be inferred via the biconditional elimination rule. u_1 -> s can be inferred via the biconditional elimination rule. ~u_1 can be inferred via the biconditional elimination rule.