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:
case H; clear H; do_intro
Level 3: evaluation returns
case H; clear H; do_intro; do_intro
where
use_hyp := <tactic closure>
of type
tacvalue
do_intro := <tactic closure>
of type
tacvalue
destruct_hyp := <tactic closure>
of type
tacvalue
H := H
of type
intropattern of type tacvalue