Goal:
  
  A : Type
  B : Type
  p : A → B → bool
  a : A
  l : list A
  h : is_true (forallb2 p (a :: l) [])
  IHl : ∀ l' : list B,
          forallb2 p l l' → Forall2 (λ (x : A) (y : B), p x y) l l'
  ============================
   (Forall2 (λ (x : A) (y : B), p x y) (a :: l) [])


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>
DEBUG LTAC Evaluated term: l'