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_1 -> q. q -> u_1. u_1 <-> q_1.
u_1 <-> q can be inferred via the biconditional introduction rule. q <-> u_1 can be inferred via the biconditional introduction rule. u_1 -> q_1 can be inferred via the biconditional elimination rule. q_1 -> u_1 can be inferred via the biconditional elimination rule.