(** ** Lemmas for list *)
Definition concat_list_string(xs:list string) :=
fold_right (fun s s' : string => (s ++ s')%string) ""%string xs.

Lemma forallb_divide_right : forall (A:Type)(f:A -> bool) (l l':list A),
forallb f (l ++ l') = true -> forallb f l' = true.