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_3 -> u. s_3 -> u. u -> s_3.
s_3 <-> u can be inferred via the biconditional introduction rule. s_3 <-> u can be inferred via the biconditional introduction rule. u <-> s_3 can be inferred via the biconditional introduction rule. u <-> s_3 can be inferred via the biconditional introduction rule.