enum_fset :> seq K;
  _ : canonical_keys enum_fset
}.on finset_of (_ : phant K) := finSet.

End Def.Definition pred_of_finset (K : choiceType)
  (f : finSet K) : pred K := fun k => k \in (Canonical fsetType := Eval hnf in [subType for (@enum_fset K)].Canonical fset_eqType := Eval hnf in EqType {fset K} fset_eqMixin.

Proof. by case: A. Qed.