Goal:
  
  G : t
  A : Type
  B : Type
  C : Type
  f : A -> B -> A
  g : C -> B
  l : list C
  acc : A
  ============================
   (fold_left f (map g l) acc =
    fold_left (fun (acc : A) (x : C) => f acc (g x)) l acc)


DEBUG:Going to execute:
induction l in acc |- *; cbn