Consider the following premises. forall y: T_4(y) -> W(y). forall y: T_4(y) <-> W(y). Can we infer forall y: W(y) <-> T_4(y) from them? If possible, name the inference rules being used at each step.
 From forall y: T_4(y) <-> W(y) we can infer forall y: W(y) -> T_4(y) via universal biconditional elimination. Finally, from forall y: W(y) -> T_4(y), forall y: T_4(y) -> W(y) we can infer forall y: W(y) <-> T_4(y) via universal biconditional introduction. Therefore, the answer is yes.