docs: def _stringify_predicate_value(value):
      """Convert Python objects to Space-Track compatible strings
  
      - Booleans (``True`` -> ``'true'``)
      - Sequences (``[25544, 34602]`` -> ``'25544,34602'``)
      - dates/datetimes (``date(2015, 12, 23)`` -> ``'2015-12-23'``)
      - ``None`` -> ``'null-val'``
      """
      if isinstance(value, bool):
          return str(value).lower()
      elif isinstance(value, Sequence) and not isinstance(value, six.string_types):
          return ','.join(_stringify_predicate_value(x) for x in value)
      elif isinstance(value, datetime.datetime):
          return value.isoformat(sep=' ')
      elif isinstance(value, datetime.date):
          return value.isoformat()
      elif value is None:
          return 'null-val'
      else:
          return str(value)