Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  p : A → B → bool
  l : list A
  l' : list B
  Hp : ∀ (x : A) (y : B), reflect (P x y) (p x y)
  ============================
   (forallb2 p l l' → Forall2 P l l')


DEBUG:Going to execute:
induction l in l' |- *; destruct l'