Goal:
  
  T : Type
  Listable_T : Listable T
  FinitelyListable_T : FinitelyListable T
  v : T
  x : T
  xs : list T
  n : nat
  Hnth : nth_error xs n = Some v
  Hfold2 : is_true
             (forallb (fun '(n, t) => n == find_index t)
                (combine (seq (S (find_index x)) (length xs)) xs))
  IHxs : BinInt.Z.of_nat (find_index v) =
         BinInt.Z.add
           (BinInt.Z.add (BinInt.Z.of_nat (find_index x)) (BinInt.Z.of_nat n))
           (BinNums.Zpos BinNums.xH)
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat (find_index x))
  cstr1 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat (find_index v))
  ============================
   (BinInt.Z.of_nat (find_index v) =
    BinInt.Z.add (BinInt.Z.of_nat (find_index x))
      (BinInt.Z.add (BinInt.Z.of_nat n) (BinNums.Zpos BinNums.xH)))


DEBUG:Going to execute:
idtac