docs: def combine_uuids(uuids, ordered=True, salt=''):
      """
      Creates a uuid that specifies a group of UUIDS
  
      Args:
          uuids (list): list of uuid objects
          ordered (bool): if False uuid order changes the resulting combined uuid
              otherwise the uuids are considered an orderless set
          salt (str): salts the resulting hash
  
      Returns:
          uuid.UUID: combined uuid
  
      CommandLine:
          python -m utool.util_hash --test-combine_uuids
  
      Example:
          >>> # ENABLE_DOCTEST
          >>> from utool.util_hash import *  # NOQA
          >>> import utool as ut
          >>> uuids = [hashable_to_uuid('one'), hashable_to_uuid('two'),
          >>>          hashable_to_uuid('three')]
          >>> combo1 = combine_uuids(uuids, ordered=True)
          >>> combo2 = combine_uuids(uuids[::-1], ordered=True)
          >>> combo3 = combine_uuids(uuids, ordered=False)
          >>> combo4 = combine_uuids(uuids[::-1], ordered=False)
          >>> result = ut.repr4([combo1, combo2, combo3, combo4], nobr=True)
          >>> print(result)
          UUID('83ee781f-8646-ccba-0ed8-13842825c12a'),
          UUID('52bbb33f-612e-2ab8-a62c-2f46e5b1edc8'),
          UUID('945cadab-e834-e581-0f74-62f106d20d81'),
          UUID('945cadab-e834-e581-0f74-62f106d20d81'),
  
      Example:
          >>> # ENABLE_DOCTEST
          >>> from utool.util_hash import *  # NOQA
          >>> import utool as ut
          >>> uuids = [uuid.UUID('5ff6b34e-7d8f-ef32-5fad-489266acd2ae'),
          >>>          uuid.UUID('f2400146-ec12-950b-1489-668228e155a8'),
          >>>          uuid.UUID('037d6f31-8c73-f961-1fe4-d616442a1e86'),
          >>>          uuid.UUID('ca45d6e2-e648-09cc-a49e-e71c6fa3b3f3')]
          >>> ordered = True
          >>> salt = u''
          >>> result = combine_uuids(uuids, ordered, salt)
          >>> print(result)
          1dabc66b-b564-676a-99b4-5cae7a9e7294
      """
      if len(uuids) == 0:
          return get_zero_uuid()
      elif len(uuids) == 1:
          return uuids[0]
      else:
          if not ordered:
              uuids = sorted(uuids)
          sep_str = '-'
          sep_byte = six.binary_type(six.b(sep_str))
          pref = six.binary_type(six.b('{}{}{}'.format(salt, sep_str, len(uuids))))
          combined_bytes = pref + sep_byte.join([u.bytes for u in uuids])
          combined_uuid = hashable_to_uuid(combined_bytes)
          return combined_uuid