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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 _serialize_date(value):
 """Serialize a Date object to its proper ISO-8601 representation."""

    if not isinstance(value, date):
        raise ValueError(u'The received object was not a date: '
                         u'{} {}'.format(type(value), value))
    return value.isoformat()