Goal:
  
  T : Type
  Listable_T : Listable T
  FinitelyListable_T : FinitelyListable T
  v : T
  x : T
  xs : list T
  n : nat
  IHxs : find_index v = S (find_index x + n)
  Hnth : nth_error xs n = Some v
  Hfold2 : is_true
             (forallb (fun '(n, t) => n == find_index t)
                (combine (seq (S (find_index x)) (length xs)) xs))
  ============================
   (find_index v = find_index x + S n)


DEBUG:Going to execute:
idtac