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|)
  ============================
   (BinInt.Z.of_nat #|l| =
    BinInt.Z.max BinNums.Z0
      (BinInt.Z.sub
         (BinInt.Z.max BinNums.Z0
            (BinInt.Z.sub
               (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
               (BinNums.Zpos BinNums.xH))) BinNums.Z0))


DEBUG:Going to execute:
fun => <coq-core.plugins.zify::ITER@0>