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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 to_iso_date(date, default=None):
 """ISO representation for the date object :param date: A date object :type field: datetime/DateTime :returns: The ISO format of the date :rtype: str """


    # not a date
    if not is_date(date):
        return default

    # handle Zope DateTime objects
    if isinstance(date, (DateTime)):
        return date.ISO8601()

    # handle python datetime objects
    return date.isoformat()