Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  n : nat
  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)
  ============================
   (nth_error (mapi_rec f l (S k)) n =
    option_map (f (S (n + k))) (nth_error l n))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2