Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  p : A → B → bool
  a : A
  l : list A
  Hp : ∀ (x : A) (y : B), reflect (P x y) (p x y)
  IHl : ∀ l' : list B, forallb2 p l l' → Forall2 P l l'
  H : is_true false
  ============================
   (Forall2 P (a :: l) [])


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.