def ParseFields(selected, default):
  """Parses the values of "--field"-like options.

  @type selected: string or None
  @param selected: User-selected options
  @type default: list
  @param default: Default fields

  """
  if selected is None:
    return default

  if selected.startswith("+"):
    return default + selected[1:].split(",")

  return selected.split(",")