Consider the following premises. ~P_8(a_7). forall x_8: P_8(x_8) -> Q_8(x_8). Q_8(a_7) or s_2. forall x_8: Q_8(x_8) -> P_8(x_8). Can we infer ~s_2 from them? If possible, name the inference rules being used at each step.
 From forall x_8: P_8(x_8) -> Q_8(x_8), forall x_8: Q_8(x_8) -> P_8(x_8) we can infer forall x_8: P_8(x_8) <-> Q_8(x_8) via universal biconditional introduction. From forall x_8: P_8(x_8) <-> Q_8(x_8) we can infer P_8(a_7) <-> Q_8(a_7) via universal instantiation. From P_8(a_7) <-> Q_8(a_7), ~P_8(a_7) we can infer ~Q_8(a_7) via biconditional elimination. Finally, from Q_8(a_7) or s_2, ~Q_8(a_7) we can infer s_2 via disjunctive syllogism, which contradicts ~s_2. Therefore, the answer is no.