code: def invalid_ipa_characters(unicode_string, indices=False):
      
      if unicode_string is None:
          return None
      if indices:
          return [(i, unicode_string[i]) for i in range(len(unicode_string)) if unicode_string[i] not in UNICODE_TO_IPA]
      return set([c for c in unicode_string if c not in UNICODE_TO_IPA])
docs: Return the list of Unicode characters
      in the given Unicode string
      that are not IPA valid.
  
      Return ``None`` if ``unicode_string`` is ``None``.
  
      :param str unicode_string: the Unicode string to be parsed
      :param bool indices: if ``True``, return a list of pairs (index, invalid character),
                           instead of a list of str (characters).
      :rtype: list of str or list of (int, str)