Goal:
  
  A : Type
  a : A
  l : list A
  l' : list A
  IHl : ∀ v : nat, #|l| ≤ v → nth_error (l ++ l') v = nth_error l' (v - #|l|)
  H : BinInt.Z.le
        (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
        BinNums.Z0
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  ============================
   (nth_error (a :: l ++ l') 0 = nth_error l' (0 - S #|l|))


DEBUG:Going to execute:
Zify.zify_iter_specs