code: def _oxford_comma_separator(i, length):
      
      if length == 1:
          return None
      elif length < 3 and i == 0:
          return ' and '
      elif i < length - 2:
          return ', '
      elif i == length - 2:
          return ', and '
      else:
          return None
docs: Make a separator for a prose-like list with `,` between items except
      for `, and` after the second to last item.