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


DEBUG:Going to execute:
apply : eq_sym