Admitted. (* Higher-Order *)
Fixpoint In {A : Type} (x : A) (l : list A) : Prop :=
match l with
| [] => False
| x' :: l' => x' = x \/ In x l'
end.

(* Specifications: *)
Lemma In_map :
forall (A B : Type) (f : A -> B) (l : list A) (x : A),
In x l ->
In (f x) (map f l).