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


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