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'
  ============================
   (forallb2 p (a :: l) [] → Forall2 P (a :: l) [])


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2