Goal:
  
  T : Type
  Listable_T : Listable T
  FinitelyListable_T : FinitelyListable T
  v : T
  x : T
  xs : list T
  IHxs : forall offset n : nat,
         forallb (fun '(n0, t) => n0 == find_index t)
           (combine (seq offset (length xs)) xs) ->
         nth_error xs n = Some v -> find_index v = offset + n
  offset : nat
  n : nat
  Hnth : nth_error (x :: xs) n = Some v
  p : offset = find_index x
  Hfold : is_true
            (forallb (fun '(n, t) => n == find_index t)
               (combine (seq (S offset) (length xs)) xs))
  ============================
   (offset + n = find_index v)


DEBUG:Going to execute:
trivial