Section Graph.
Variable (A : finType) (B : Type) (f : A -> B).
Set Default Proof Using "Type".
Definition graph_of_fun : list (A*B) := map (fun x => (x, f x)) enum.

Lemma lookup_In (X : eqType) (Y : Type) (lst : list (X*Y)) x y :
(x, y) el lst ->
exists y', lookup x lst = Some y'.