Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  x : A
  y : B
  l : list A
  r : list B
  r0 : R n x y
  h : All2i R (S n) l r
  IHh : All2i (λ (i : nat) (x : A) (y : B), R (S n + #|l| - S i) x y) 0
          (List.rev l) (List.rev r)
  i : nat
  x0 : A
  y0 : B
  X : R (n + #|l| - i) x0 y0
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat i)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  cstr1 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  z2 : BinNums.Z
  H2 : BinInt.Z.lt BinNums.Z0
         (BinInt.Z.sub
            (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.of_nat i) (BinNums.Zpos BinNums.xH)))
       ∧ z2 =
         BinInt.Z.sub
           (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.of_nat i) (BinNums.Zpos BinNums.xH))
       ∨ BinInt.Z.le
           (BinInt.Z.sub
              (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.of_nat i) (BinNums.Zpos BinNums.xH)))
           BinNums.Z0 ∧ z2 = BinNums.Z0
  z3 : BinNums.Z
  H3 : BinInt.Z.lt BinNums.Z0
         (BinInt.Z.sub
            (BinInt.Z.add (BinInt.Z.of_nat n) (BinInt.Z.of_nat #|l|))
            (BinInt.Z.of_nat i))
       ∧ z3 =
         BinInt.Z.sub
           (BinInt.Z.add (BinInt.Z.of_nat n) (BinInt.Z.of_nat #|l|))
           (BinInt.Z.of_nat i)
       ∨ BinInt.Z.le
           (BinInt.Z.sub
              (BinInt.Z.add (BinInt.Z.of_nat n) (BinInt.Z.of_nat #|l|))
              (BinInt.Z.of_nat i)) BinNums.Z0 ∧ z3 = BinNums.Z0
  ============================
   (z3 = z2)


DEBUG:Going to execute:
zify_saturate