Section Bool.
Variable A : Type.
Variable f : A -> bool.
(** find whether a boolean function can be satisfied by an elements of the list. *)
Fixpoint existsb (l:list A) : bool :=
match l with
| nil => false
| a::l => f a || existsb l
end.
(** find whether a boolean function is satisfied by all the elements of a list. *)
Fixpoint forallb (l:list A) : bool :=
match l with
| nil => true
| a::l => f a && forallb l
end.

Lemma forallb_forall :
forall l, forallb l = true <-> (forall x, In x l -> f x = true).