code: def add_mark_at(string, index, mark):
      
      if index == -1:
          return string
      # Python can handle the case which index is out of range of given string
      return string[:index] + add_mark_char(string[index], mark) + string[index+1:]
docs: Add mark to the index-th character of the given string. Return the new string after applying change.
      Notice: index > 0