Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  n : nat
  x : A
  Hnth : nth_error (a :: l) (S n) = Some x
  HPl : is_true (forallb p (a :: l))
  IHl : ∀ n : nat, nth_error l n = Some x → forallb p l → p x
  ============================
   (is_true (p x))


DEBUG:Going to execute:
DepElim.noconf H
Level 2: evaluation returns
CoreTactics.block_goal; CoreTactics.revert_until H; CoreTactics.block_goal;
 CoreTactics.on_last_hyp ltac:(fun H' => revert H');
 DepElim.simplify_dep_elim; CoreTactics.intros_until_block;
 CoreTactics.intros_until_block
where
H := Hnth
of type
constr of type tacvalue

Level 1: evaluation returns
CoreTactics.block_goal; CoreTactics.revert_until H; CoreTactics.block_goal;
 CoreTactics.on_last_hyp ltac:(fun H' => revert H');
 DepElim.simplify_dep_elim; CoreTactics.intros_until_block;
 CoreTactics.intros_until_block
where
H := Hnth
of type
constr of type tacvalue