Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  n : nat
  x : A
  Hnth : nth_error (a :: l) (S n) = Some x
  ============================
   (let H := CoreTactics.block in
    forallb p (a :: l)
    → (∀ n : nat, nth_error l n = Some x → forallb p l → p x)
      → let H0 := CoreTactics.block in p x)


DEBUG:Going to execute:
CoreTactics.on_last_hyp ltac:(fun H' => revert H')
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            forallb p (a :: l)
                            → (∀ n : nat,
                                 nth_error l n = Some x → forallb p l → p x)
                              → let H0 := CoreTactics.block in p x)