| (pos n) \Rightarrow (neg (pred ((S m) * (S n))))
| (neg n) \Rightarrow (pos (pred ((S m) * (S n))))]].
| (pos n) \Rightarrow (neg (pred ((S m) * (S n))))
| (neg n) \Rightarrow (pos (pred ((S m) * (S n))))]].
variant distr_Ztimes_Zplus: \forall x,y,z.
x * (y + z) = x*y + x*z \def
distributive_Ztimes_Zplus.
variant distr_Ztimes_Zplus: \forall x,y,z.
x * (y + z) = x*y + x*z \def
distributive_Ztimes_Zplus.