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: p_8 -> u_4. u_4 -> p_8. p_8 -> u_4.
p_8 <-> u_4 can be inferred via the biconditional introduction rule. u_4 <-> p_8 can be inferred via the biconditional introduction rule. u_4 <-> p_8 can be inferred via the biconditional introduction rule. p_8 <-> u_4 can be inferred via the biconditional introduction rule.