Goal:
  
  G : t
  A : Type
  B : Type
  f : A -> B -> A
  g : A -> B -> A
  hfg : f =2 g
  a : B
  x : list B
  acc : A
  IHx : forall acc : A, fold_left f x acc = fold_left g x acc
  ============================
   (fold_left f (a :: x) acc = fold_left g (a :: x) acc)


DEBUG:Going to execute:
trivial; hnf