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 or s_1. u_4 or ~r_4. s_1 <-> u_4.
r_4 -> u_4 can be inferred via the rewriting rule. s_1 -> u_4 can be inferred via the biconditional elimination rule. u_4 -> s_1 can be inferred via the biconditional elimination rule.