Goal:
  
  T : Type
  Listable_T : Listable T
  FinitelyListable_T : FinitelyListable T
  v : T
  x : T
  xs : list T
  IHxs : forall offset n : nat,
         forallb (fun '(n0, t) => n0 == find_index t)
           (combine (seq offset (length xs)) xs) ->
         nth_error xs n = Some v -> find_index v = offset + n
  offset : nat
  n : nat
  Hnth : nth_error (x :: xs) n = Some v
  p : offset = find_index x
  Hfold : is_true
            (forallb (fun '(n, t) => n == find_index t)
               (combine (seq (S offset) (length xs)) xs))
  ============================
   (find_index v = offset + n)


DEBUG:Going to execute:
case not_locked_false_eq_true; Ltac.assumption
Level 1: Backtrace:

In environment
T : Type
Listable_T : Listable T
FinitelyListable_T : FinitelyListable T
v, x : T
xs : list T
IHxs : forall offset n : nat,
       forallb (fun '(n0, t) => n0 == find_index t)
         (combine (seq offset (length xs)) xs) ->
       nth_error xs n = Some v -> find_index v = offset + n
offset, n : nat
Hnth : nth_error (x :: xs) n = Some v
p : offset = find_index x
Hfold : forallb (fun '(n, t) => n == find_index t)
          (combine (seq (S offset) (length xs)) xs)
Unable to unify "offset + n" with "find_index v".
Level 0: Backtrace:

In environment
T : Type
Listable_T : Listable T
FinitelyListable_T : FinitelyListable T
v, x : T
xs : list T
IHxs : forall offset n : nat,
       forallb (fun '(n0, t) => n0 == find_index t)
         (combine (seq offset (length xs)) xs) ->
       nth_error xs n = Some v -> find_index v = offset + n
offset, n : nat
Hnth : nth_error (x :: xs) n = Some v
p : offset = find_index x
Hfold : forallb (fun '(n, t) => n == find_index t)
          (combine (seq (S offset) (length xs)) xs)
Unable to unify "offset + n" with "find_index v".
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.