Inductive member (a : T) : list T -> Type :=
  | MZ : forall ls, member a (a :: ls)
  | MN : forall l ls, member a ls -> member a (l :: ls).Inductive member (a : T) : list T -> Type :=
  | MZ : forall ls, member a (a :: ls)
  | MN : forall l ls, member a ls -> member a (l :: ls).Inductive member (a : T) : list T -> Type :=
  | MZ : forall ls, member a (a :: ls)
  | MN : forall l ls, member a ls -> member a (l :: ls).Fixpoint to_nat {ls} (m : member a ls) : nat :=
      match m with
        | MZ _ => 0
        | MN _ _ m => S (to_nat m)
      end.

Lemma member_case : forall x ls (m : member x ls), match ls as ls return member x ls -> Prop with | nil => fun _ => False | l :: ls' => fun m => (exists (pf : l = x), m = match pf in _ = z return member z (l :: ls') with | eq_refl => MZ _ ls' end) \/ exists m' : member x ls', m = MN _ m' end m.