Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  IHl : ∀ n : nat, nth_error l n = Some a → forallb p l → p a
  H : p a ∧ forallb p l
  ============================
   (is_true (p a))


DEBUG:Going to execute:
fun H => case H; clear H; do_intro; do_intro