Goal:
  
  A : Type
  P : nat → A → Type
  x : A
  l : list A
  IHl : ∀ k : nat,
          Alli P k l
          → Alli (λ k' : nat, P (Nat.pred #|l| - k' + k)) 0 (List.rev l)
  k : nat
  a : Alli P k l
  b : Alli P (#|l| + k) [x]
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  z0 : BinNums.Z
  H0 : BinInt.Z.lt BinNums.Z0
         (BinInt.Z.sub
            (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
            (BinNums.Zpos BinNums.xH))
       ∧ z0 =
         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 ∧ 
         z0 = BinNums.Z0
  z1 : BinNums.Z
  H1 : BinInt.Z.lt BinNums.Z0 (BinInt.Z.sub z0 BinNums.Z0)
       ∧ z1 = BinInt.Z.sub z0 BinNums.Z0
       ∨ BinInt.Z.le (BinInt.Z.sub z0 BinNums.Z0) BinNums.Z0
         ∧ z1 = BinNums.Z0
  ============================
   (BinInt.Z.of_nat #|l| = z1)


DEBUG:Going to execute:
idtac