/** The key of the configuration. e.g. lang */
    String INDEX_FIELD_LANG = "index.field.lang";

    /** The key of the configuration. e.g. has_cache */
    String INDEX_FIELD_has_cache = "index.field.has_cache";

    /** The key of the configuration. e.g. last_modified */
    String INDEX_FIELD_last_modified = "index.field.last_modified";

    /** The key of the configuration. e.g. anchor */