Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  l2 : list A
  r2 : list B
  n : nat
  x : A
  y : B
  l : list A
  r : list B
  r0 : R n x y
  h1 : All2i R (S n) l r
  h2 : All2i R (n + S #|l|) l2 r2
  IHh1 : ∀ (l2 : list A) (r2 : list B),
           All2i R (S (n + #|l|)) l2 r2 → All2i R (S n) (l ++ l2) (r ++ r2)
  ============================
   (n + S #|l| = S (n + #|l|))


DEBUG:Going to execute:
intros; Zify.zify_pre_hook; zify_elim_let; zify_op;
            Zify.zify_iter_specs