Goal:
  
  A : Type
  P : A → Prop
  a : A
  l : list A
  p : A → bool
  IHl : ∀ p : A → bool, forallb p l → (∀ x : A, p x → P x) → Forall P l
  H : is_true (p a && forallb p l)
  H0 : ∀ x : A, p x → P x
  ============================
   (Forall P (a :: l))


DEBUG:Going to execute:
constructor; eauto