Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  x : A
  l : list A
  IHl : ∀ n : nat,
          mapi_rec f (List.rev l) n =
          List.rev (mapi_rec (λ (k : nat) (x : A), f (#|l| + n - S k) x) l 0)
  n : nat
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  ============================
   (BinInt.Z.of_nat n =
    BinInt.Z.max BinNums.Z0
      (BinInt.Z.sub
         (BinInt.Z.add
            (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
            (BinInt.Z.of_nat n))
         (BinInt.Z.add (BinInt.Z.add (BinInt.Z.of_nat #|l|) BinNums.Z0)
            (BinNums.Zpos BinNums.xH))))


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