Goal:
    A : Type  p : A → bool  a : A  l : list A  x : A
  ============================
   (nth_error (a :: l) 0 = 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:
simplify *