CoFixpoint const  : Stream A := Cons a const.CoFixpoint zipWith (a:Stream A) (b:Stream B) : Stream C :=
Cons (f (hd a) (hd b)) (zipWith (tl a) (tl b)).

Lemma Str_nth_tl_zipWith : forall n (a:Stream A) (b:Stream B), Str_nth_tl n (zipWith a b)= zipWith (Str_nth_tl n a) (Str_nth_tl n b).