Definition fsfun_of_can_ffun (T : {fset K}) (g : {ffun T -> V})
(can_g : forall k : T,  g k != default (val k)) :=
@Fsfun K V default (FinMap g) (appP forallP idP can_g).

(* Specifications: *)
Lemma fsfun_of_can_ffunE (T : {fset K}) (g : {ffun T -> V})
(can_g : forall k : T ,  g k != default (val k)) k (kg : k \in T) :
(fsfun_of_can_ffun can_g) k = g.[kg].