Consider the following premises. r <-> t. forall y: R(y) <-> S(y). ~S(a_12). R(a_12) <-> r. Can we infer t from them? If possible, name the inference rules being used at each step.
 From forall y: R(y) <-> S(y) we can infer R(a_12) <-> S(a_12) via universal instantiation. From R(a_12) <-> S(a_12), ~S(a_12) we can infer ~R(a_12) via biconditional elimination. From R(a_12) <-> r, ~R(a_12) we can infer ~r via biconditional elimination. Finally, from r <-> t, ~r we can infer ~t via biconditional elimination, which contradicts t. Therefore, the answer is no.