Goal:
  
  A : Type
  n : nat
  a : A
  l : list A
  l' : list A
  IHl : ∀ (n : nat) (l' : list A),
          n ≤ #|l| → skipn n (l ++ l') = skipn n l ++ l'
  Hn : BinInt.Z.le
         (BinInt.Z.add (BinInt.Z.of_nat n) (BinNums.Zpos BinNums.xH))
         (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  ============================
   (BinInt.Z.le (BinInt.Z.of_nat n) (BinInt.Z.of_nat #|l|))


DEBUG:Going to execute:
idtac