Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  x : A
  Hnth : nth_error (a :: l) 0 = Some x
  ============================
   (forallb p (a :: l)
    → (∀ n : nat, nth_error l n = Some x → forallb p l → p x)
      → let H := CoreTactics.block in p x)


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end