Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  g : nat → A → B
  l : list A
  H : ∀ (n : nat) (x : A), f n x = g n x
  ============================
   (∀ (k : nat) (x : A), 0 ≤ k → k < #|l| + 0 → f k x = g k x)


DEBUG:Going to execute:
easy
DEBUG LTAC Evaluated term: mapi_rec_ext