Canonical tag_countType := Eval hnf in CountType {i : I & T_ i} tag_countMixin.
End TagCountType.
(* The remaining Canonicals for standard datatypes. *)
Section CountableDataTypes.
Implicit Type T : countType.

Lemma nat_pickleK : pcancel id (@Some nat). Proof. by []. Qed.
Definition nat_countMixin := CountMixin nat_pickleK.