Goal:
  
  A : Type
  n : nat
  l' : list A
  Hn : BinInt.Z.le
         (BinInt.Z.add (BinInt.Z.of_nat n) (BinNums.Zpos BinNums.xH))
         BinNums.Z0
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  ============================
   (skipn (S n) l' = skipn (S n) [] ++ l')


DEBUG:Going to execute:
idtac