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


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in _ => intros _
           | |- _ => try (intro; CoreTactics.intros_until_block)
           end