Goal:
  
  T : Type
  Listable_T : Listable T
  FinitelyListable_T : FinitelyListable T
  n : nat
  m : nat
  x : T
  y : T
  Hn : nth_error (list_all T) n = Some x
  Hm : nth_error (list_all T) m = Some y
  Hxy : x = y
  ============================
   (n = m)


DEBUG:Going to execute:
rewrite <- !find_index_iff in *; subst