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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)