Section Facts.
Variable A : Type.
(** *** Generic facts *)
(** Destruction *)
(** *** Head and tail *)
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(** *** Facts about [In] *)
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(** Characterization of [In] *)

(** Inversion *)
Lemma in_inv : forall (a b:A) (l:list A), In b (a :: l) -> a = b \/ In b l.