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)
  xs : list A
  ys : list B
  x : A
  y : B
  H : Forall2 R (x :: xs) (y :: ys)
  H' : sigP (λ v : R x y ∧ Forall2 R xs ys, Forall2_cons x y v.p1 v.p2 = H)
  Hc : P (x :: xs) (y :: ys) (Forall2_cons x y (projP1 H').p1 (projP1 H').p2)
  F : ∀ a : Forall2 R xs ys, P xs ys a
  ============================
   (P (x :: xs) (y :: ys) (Forall2_cons x y (projP1 H').p1 (projP1 H').p2))


DEBUG:Going to execute:
<coq-core.plugins.ltac::exact@0> $1
DEBUG LTAC Evaluated term: (Hc x y xs ys (projP1 H').p1 
                              (projP1 H').p2 (F (projP1 H').p2))
DEBUG LTAC Evaluated term: (projP2 H')