Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  R' : ∀ (a : A) (b : B), R a b → Prop
  P : ∀ (x : list A) (y : list B) (a : Forall2 R x y),
        Forall2_dep R' a → Type
  Hn : P [] [] (Forall2_nil R) Forall2_dep_nil
  Hc : ∀ (x : A) (y : B) (l : list A) (l' : list B) 
         (r : R x y) (rs : Forall2 R l l') (r' : R' x y r) 
         (a : Forall2_dep R' rs),
         P l l' rs a
         → P (x :: l) (y :: l') (Forall2_cons x y r rs)
             (Forall2_dep_cons r' a)
  a' : Forall2_dep R' (Forall2_nil R)
  ============================
   (Forall2_dep_nil = a' → P [] [] (Forall2_nil R) a')


DEBUG:Going to execute:
intro; subst