Consider the following premises. r_2 -> r_1. r_2 <-> r_1. r_1 -> r_2. Can we infer r_1 <-> r_2 from them? If possible, name the inference rules being used at each step.
 From r_2 -> r_1, r_1 -> r_2 we can infer r_2 <-> r_1 via biconditional introduction. From r_2 <-> r_1 we can infer r_2 -> r_1 via biconditional elimination. From r_2 <-> r_1 we can infer r_1 -> r_2 via biconditional elimination. Finally, from r_1 -> r_2, r_2 -> r_1 we can infer r_1 <-> r_2 via biconditional introduction. Therefore, the answer is yes.