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


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2