/**
 * This function processe the zotero extra field.
 * It sorts the dois and keep the first one as is and prefix the rest with 'previous'.
 * It also add kerkoCite.ItemAlsoKnownAs: line and sort the dois and kerkoCite.ItemAlsoKnownAs: line.
 *
 * @param input - The extra field as a string. separated by newline.
 * @returns - The extra field as a string. separated by newline.
 */
export default function processExtraField(input?: string): string;