Instance Fin_eq_dec n : eq_dec (Fin.t n).
(* (** * finTypes from Lists  *) *)
(* (* Conversion of lists over eqTypes to finite types *) *)
(* (* *) *)
(** * Pure predicates  *)
(* taken from the development of Herditarily finite sets by Prof. Smolka and Kathrin Stark. *)
(* *)
Definition pure (X:Type) (p: X -> Prop) {D:forall x, dec (p x)} x := if Dec (p x) then True else False.
Arguments pure {X} p {D} x.
Ltac impurify H := pose proof (pure_impure H) as impureH; try (clear H; rename impureH into H).
Arguments purify {X} {p} {D} {x} px.
(* (** * Definition of subtypes *) *)
Definition subtype {X:Type} (p: X -> Prop) {D: forall x, dec (p x)} := { x:X | pure p x}.
Arguments subtype {X} p {D}.
Instance subType_eq_dec X (_:eq_dec X) (p: X -> Prop) (_: forall x, dec (p x)): eq_dec (subtype p).
(* Lemma proj1_sig_fun (X: eqType) (p: X -> Prop) (x x': X) (p1: p x) (p2: p x'): exist p x p1 = exist p x' p2 -> x = x'. *)
(* Proof. *)
(*   intro E. change x with (proj1_sig (exist p x p1)). change x' with (proj1_sig (exist p x' p2)). *)
(*   now inv E. *)
(* Qed. *)
(* (** * Subtypes from lists *) *)
(* Lemma in_undup_iff (X: eqType) (A: list X) x : x el A <-> x el (undup A). *)
(* Proof. *)
(*   now rewrite undup_id_equi. *)
(* Qed. *)
Fixpoint toSubList (X: Type) (A: list X) (p: X -> Prop) (D:forall x, dec (p x))  : list (subtype p) :=
match A with
| nil => nil
| cons x A' => match Dec (p x) with
| left px => (exist  _ x (purify px)) :: toSubList A' D
| right _ => toSubList  A' _ end
end.
Arguments toSubList {X} A p {D}.

Lemma toSubList_count (X: eqType) (p: X -> Prop) (A: list X) (_:forall x, dec (p x)) x:
count (toSubList A p) x = count A (proj1_sig x).