Goal:
  
  T : Type
  Listable_T : Listable T
  FinitelyListable_T : FinitelyListable T
  v : T
  offset : nat
  n : nat
  Hfold : is_true
            (forallb (fun '(n, t) => n == find_index t)
               (combine (seq offset (length [])) []))
  Hnth : nth_error [] n = Some v
  ============================
   (find_index v = offset + n)


DEBUG:Going to execute:
destruct n; cbn in *