(** [(list_power x y)] is [y^x], or the set of sequences of elts of [y] indexed by elts of [x], sorted in lexicographic order. *)
Fixpoint list_power (A B:Type)(l:list A) (l':list B) :
list (list (A * B)) :=
match l with
| [] => [[]]
| x :: l =>
flat_map (fun f:list (A * B) => map (fun y:B => (x, y) :: f) l')
(list_power l l')
end.
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(** ** Boolean operations over lists *)
(*************************************)
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
| [] => false
| a :: l => f a || existsb l
end.

Lemma existsb_nth : forall l n d, n < length l ->
existsb l = false -> f (nth n l d) = false.