Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  n : nat
  x : A
  Hnth : nth_error (a :: l) (S n) = Some x
  HPl : is_true (forallb p (a :: l))
  IHl : ∀ n : nat, nth_error l n = Some x → forallb p l → p x
  ============================
   (is_true (p x))


DEBUG:Going to execute:
match goal with
           | |- ?T => change (let _ := CoreTactics.block in T)
           end