Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  k : nat
  a : A
  l : list A
  IHl : ∀ n k : nat,
          nth_error (mapi_rec f l k) n =
          option_map (f (n + k)) (nth_error l n)
  ============================
   (Some (f k a) = Some (f k a))


DEBUG:Going to execute:
PTRDEBUGTACreflexivity