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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 create(cls, name, ne_ref=None, operator='exclusion', sub_expression=None, comment=None):
 """ Create the expression :param str name: name of expression :param list ne_ref: network element references for expression :param str operator: 'exclusion' (negation), 'union', 'intersection' (default: exclusion) :param dict sub_expression: sub expression used :param str comment: optional comment :raises CreateElementFailed: element creation failed with reason :return: instance with meta :rtype: Expression """

        sub_expression = [] if sub_expression is None else [sub_expression]
        json = {'name': name,
                'operator': operator,
                'ne_ref': ne_ref,
                'sub_expression': sub_expression,
                'comment': comment}

        return ElementCreator(cls, json)