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


DEBUG:Going to execute:
subst
DEBUG LTAC Evaluated term: find_index_iff
DEBUG LTAC Evaluated term: find_index_iff
DEBUG LTAC Evaluated term: find_index_iff