Goal:
  
  A : Type
  B : Type
  f : A → B
  IHn : ∀ l : list A, nth_error (map f l) 0 = option_map f (nth_error l 0)
  l : list A
  ============================
   (match map f l with
    | _ :: x :: _ => Some x
    | _ => None
    end = option_map f match l with
                       | _ :: x :: _ => Some x
                       | _ => None
                       end)


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