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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 dataset_create_version(self, folder, version_notes, quiet=False, convert_to_csv=True, delete_old_versions=False, dir_mode='skip'):
 """ create a version of a dataset Parameters ========== folder: the folder with the dataset configuration / data files version_notes: notes to add for the version quiet: suppress verbose output (default is False) convert_to_csv: on upload, if data should be converted to csv delete_old_versions: if True, do that (default False) dir_mode: What to do with directories: "skip" - ignore; "zip" - compress and upload """

        if not os.path.isdir(folder):
            raise ValueError('Invalid folder: ' + folder)

        meta_file = self.get_dataset_metadata_file(folder)

        # read json
        with open(meta_file) as f:
            meta_data = json.load(f)
        ref = self.get_or_default(meta_data, 'id', None)
        id_no = self.get_or_default(meta_data, 'id_no', None)
        if not ref and not id_no:
            raise ValueError('ID or slug must be specified in the metadata')

        subtitle = meta_data.get('subtitle')
        if subtitle and (len(subtitle) < 20 or len(subtitle) > 80):
            raise ValueError(
                'Subtitle length must be between 20 and 80 characters')
        resources = meta_data.get('resources')
        if resources:
            self.validate_resources(folder, resources)

        description = meta_data.get('description')
        keywords = self.get_or_default(meta_data, 'keywords', [])

        request = DatasetNewVersionRequest(
            version_notes=version_notes,
            subtitle=subtitle,
            description=description,
            files=[],
            convert_to_csv=convert_to_csv,
            category_ids=keywords,
            delete_old_versions=delete_old_versions)
        self.upload_files(request, resources, folder, quiet, dir_mode)

        if id_no:
            result = DatasetNewVersionResponse(
                self.process_response(
                    self.datasets_create_version_by_id_with_http_info(
                        id_no, request)))
        else:
            if ref == self.config_values[
                    self.CONFIG_NAME_USER] + '/INSERT_SLUG_HERE':
                raise ValueError(
                    'Default slug detected, please change values before '
                    'uploading')
            self.validate_dataset_string(ref)
            ref_list = ref.split('/')
            owner_slug = ref_list[0]
            dataset_slug = ref_list[1]
            result = DatasetNewVersionResponse(
                self.process_response(
                    self.datasets_create_version_with_http_info(
                        owner_slug, dataset_slug, request)))

        return result