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)
  xs : list A
  ys : list B
  F : ∀ a : Forall2 R xs ys, P xs ys a
  x : A
  y : B
  H : Forall2 R (x :: xs) (y :: ys)
  ============================
   (sigP (λ v : R x y ∧ Forall2 R xs ys, Forall2_cons x y v.p1 v.p2 = H)
    → P (x :: xs) (y :: ys) H)


DEBUG:Going to execute:
destruct 1