(** find whether a boolean function is satisfied by all the elements of a list. *)
Fixpoint forallb (l:list A) : bool :=
match l with
| [] => true
| a::l => f a && forallb l
end.

Lemma forallb_app :
forall l1 l2, forallb (l1++l2) = forallb l1 && forallb l2.