Goal:
  
  T : Type
  Listable_T : Listable T
  FinitelyListable_T : FinitelyListable T
  x : T
  n : nat
  ============================
   (nth_error (list_all T) n = Some x -> find_index x = n)


DEBUG:Going to execute:
idtac
DEBUG LTAC Evaluated term: find_index_ok