code: def create(cls, name, ne_ref=None, operator='exclusion',
                 sub_expression=None, comment=None):
          
          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)
docs: 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