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_2. s -> u_1. s <-> p. p -> u_2. u_1 -> s.
~p can be inferred via the modus tollens rule. s <-> u_1 can be inferred via the biconditional introduction rule. u_1 <-> s can be inferred via the biconditional introduction rule. s -> p can be inferred via the biconditional elimination rule. p -> s can be inferred via the biconditional elimination rule.