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


DEBUG:Going to execute:
cbv beta