Goal:
  
  A : Type
  B : Type
  C : Type
  g : nat → B → C
  f : nat → A → B
  k : nat
  l : list A
  ============================
   (mapi_rec g (mapi_rec f l k) k =
    mapi_rec (λ (k : nat) (x : A), g k (f k x)) l k)


DEBUG:Going to execute:
induction l in k |- *; simpl