Goal:
  
  A : Type
  P : A → bool
  a : A
  l : list A
  n : nat
  IHl : ∀ n : nat,
          forallb P l → on_Some_or_None (λ x : A, P x) (nth_error l n)
  H : Forall (λ x : A, P x) (a :: l)
  ============================
   (on_Some_or_None (λ x : A, P x) (nth_error (a :: l) n))


DEBUG:Going to execute:
fun H => inversion_clear H