(* destruct a Fin n for n statically known into n branches *)
Ltac fin_destruct i :=
match goal with
| [ i : Fin 0 |- _ ] => destruct i
| [ i : Fin (S ?n) |- _ ] => destruct i as [[]|j]; [|fin_destruct j]
| _ => fail "no match"
end.
Arguments card _ {_}.
Definition enum {X} `{Finite X} : list X :=
map from_fin (Fin_enum (card X)).

Lemma enum_correct {X} `{Finite X} : forall (x : X), In x enum.
Definition count {X} `{Finite X} (p : X -> bool) : nat :=
length (filter p enum).