Goal:
  
  A : Type
  B : Type
  f : A → B
  IHn : nat
  l : ∀ l : list A,
        nth_error (map f l) (S IHn) = option_map f (nth_error l (S IHn))
  ============================
   (∀ l : list A,
      match map f l with
      | _ :: _ :: l1 => nth_error l1 IHn
      | _ => None
      end =
      option_map f
        match l with
        | _ :: _ :: l1 => nth_error l1 IHn
        | _ => None
        end)


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