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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 fields(self, fields):
 """Sets `sysparm_fields` after joining the given list of `fields` :param fields: List of fields to include in the response :raise: :InvalidUsage: if fields is of an unexpected type """


        if not isinstance(fields, list):
            raise InvalidUsage('fields must be of type `list`')

        self._sysparms['sysparm_fields'] = ",".join(fields)