Goal:
  
  A : Type
  P : nat → A → Type
  k : nat
  a : A
  l : list A
  i : nat
  x : A
  h : P (k + S i) x
  IHl : ∀ k : nat,
          (∀ (i : nat) (x : A), nth_error l i = Some x → P (k + i) x)
          → Alli P k l
  H : nth_error l i = Some x
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat i)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat k)
  ============================
   (BinInt.Z.add (BinInt.Z.of_nat k)
      (BinInt.Z.add (BinInt.Z.of_nat i) (BinNums.Zpos BinNums.xH)) =
    BinInt.Z.add (BinInt.Z.add (BinInt.Z.of_nat k) (BinInt.Z.of_nat i))
      (BinNums.Zpos BinNums.xH))


DEBUG:Going to execute:
constr:(false)