Section ChoiceKeys.
Variable (K : choiceType).
Implicit Types (k : K) (ks : seq K).
Definition fset_sub_pickle x := index x fset_sub_enum.
Definition fset_sub_unpickle n := nth None (map some fset_sub_enum) n.

Lemma fset_sub_pickleK : pcancel fset_sub_pickle fset_sub_unpickle.