ite /fset_sub_unpickle => x.
by rewrite (nth_map x) ?nth_index ?index_mem ?mem_fset_sub_enum.
Qed.Definition fset_sub_countMixin := CountMixin fset_sub_pickleK.
Canonical fset_sub_countType := Eval hnf in CountType fset_sub_type fset_sub_countMixin.Definition fset_sub_finMixin :=te cardE enum_fsetE size_map. Qed.

End FinTypeSet.Delimit Scope fset_scope with fset.
Local Open Scope fset_scope.Notation "#|` A |" := (size (enum_fset A))f_finset : simpl never.

Section SeqFset.

(* Specifications: *)
Proof. by move=> a; rewrite [seq_fset]unlock sort_keysE.
Proof. by rewrite [seq_fset]unlock /= siz
Proof. by rewrite [seq_fset]unlock /= sort_keys_uniq.