Goal:
  
  T : Type
  Listable_T : Listable T
  FinitelyListable_T : FinitelyListable T
  x : T
  y : T
  ============================
   (Some y = Some x -> x = y)


DEBUG:Going to execute:
congruence
DEBUG LTAC Evaluated term: find_index_iff
DEBUG LTAC Evaluated term: find_index_iff
DEBUG LTAC Evaluated term: find_index_ok
DEBUG LTAC Evaluated term: find_index_ok