Goal:
  
  A : Type
  P : nat → A → Type
  i : nat
  x : A
  n : nat
  hd : A
  tl : list A
  p : P n hd
  h : Alli P (S n) tl
  e : P (S n + i) x
  IHh : ∀ (i : nat) (x : A), nth_error tl i = Some x → P (S n + i) x
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat i)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  ============================
   (BinInt.Z.add (BinInt.Z.add (BinInt.Z.of_nat n) (BinNums.Zpos BinNums.xH))
      (BinInt.Z.of_nat i) =
    BinInt.Z.add (BinInt.Z.of_nat n)
      (BinInt.Z.add (BinInt.Z.of_nat i) (BinNums.Zpos BinNums.xH)))


DEBUG:Going to execute:
constr:(false)