Goal:
  
  A : Type
  P : nat → A → Type
  x : A
  l : list A
  k : nat
  IHl : Alli (λ k' : nat, P (Nat.pred #|l| - k' + k)) 0 (List.rev l)
  a : Alli P k l
  b : Alli P (#|l| + k) [x]
  n : nat
  x0 : A
  X : P (Nat.pred #|l| - n + k) x0
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  z1 : BinNums.Z
  H1 : BinInt.Z.lt BinNums.Z0
         (BinInt.Z.sub
            (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
            (BinNums.Zpos BinNums.xH))
       ∧ z1 =
         BinInt.Z.sub
           (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
           (BinNums.Zpos BinNums.xH)
       ∨ BinInt.Z.le
           (BinInt.Z.sub
              (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
              (BinNums.Zpos BinNums.xH)) BinNums.Z0 ∧ 
         z1 = BinNums.Z0
  z2 : BinNums.Z
  H2 : BinInt.Z.lt BinNums.Z0
         (BinInt.Z.sub z1
            (BinInt.Z.add (BinInt.Z.of_nat n) (BinNums.Zpos BinNums.xH)))
       ∧ z2 =
         BinInt.Z.sub z1
           (BinInt.Z.add (BinInt.Z.of_nat n) (BinNums.Zpos BinNums.xH))
       ∨ BinInt.Z.le
           (BinInt.Z.sub z1
              (BinInt.Z.add (BinInt.Z.of_nat n) (BinNums.Zpos BinNums.xH)))
           BinNums.Z0 ∧ z2 = BinNums.Z0
  z3 : BinNums.Z
  H3 : BinInt.Z.lt BinNums.Z0
         (BinInt.Z.sub (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
       ∧ z3 = BinInt.Z.sub (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH)
       ∨ BinInt.Z.le
           (BinInt.Z.sub (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
           BinNums.Z0 ∧ z3 = BinNums.Z0
  z4 : BinNums.Z
  H4 : BinInt.Z.lt BinNums.Z0 (BinInt.Z.sub z3 (BinInt.Z.of_nat n))
       ∧ z4 = BinInt.Z.sub z3 (BinInt.Z.of_nat n)
       ∨ BinInt.Z.le (BinInt.Z.sub z3 (BinInt.Z.of_nat n)) BinNums.Z0
         ∧ z4 = BinNums.Z0
  ============================
   (z4 = z2)


DEBUG:Going to execute:
idtac