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: s_2 or s_3. s_2 -> u_1. u_1 -> s_2. u_1 or ~s_3.
s_3 -> u_1 can be inferred via the rewriting rule. s_2 <-> u_1 can be inferred via the biconditional introduction rule. u_1 <-> s_2 can be inferred via the biconditional introduction rule.