Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  g : nat → A → B
  l : list A
  n : nat
  Hfg : ∀ (k : nat) (x : A), n ≤ k → k < #|l| + n → f k x = g k x
  ============================
   (mapi_rec f l n = mapi_rec g l n)


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