def cat_charsets(cs):
  """Combine a set into an alphabetically sorted list in written English,
  using commas and 'and'. """
  d = sorted(cs)
  if len(d) > 2:
    d[-1] = "and " + d[-1]
    d = ", ".join(d)
  else:
    d = " and ".join(d)
  return d