Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  P : ∀ (x : list A) (y : list B), Forall2 R x y → Type
  Hn : P [] [] (Forall2_nil R)
  Hc : ∀ (x : A) (y : B) (l : list A) (l' : list B) 
         (r : R x y) (a : Forall2 R l l'),
         P l l' a → P (x :: l) (y :: l') (Forall2_cons x y r a)
  F : ∀ (l : list A) (l' : list B) (a : Forall2 R l l'), P l l' a
  H : Forall2 R [] []
  ============================
   (P [] [] H)


DEBUG:Going to execute:
clear F
Level 0: Backtrace:

The variable xs was not found in the current environment.
Level 0: Backtrace:

The variable xs was not found in the current environment.