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)
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  ============================
   (BinInt.Z.add (BinInt.Z.of_nat n)
      (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH)) =
    BinInt.Z.add (BinInt.Z.add (BinInt.Z.of_nat n) (BinInt.Z.of_nat #|l|))
      (BinNums.Zpos BinNums.xH))


DEBUG:Going to execute:
Zify.zify_post_hook