* The value is, e.g. url <br>
     * comment: Field name for URL in the index.
     * @return The value of found property. (NotNull: if not found, exception but basically no way)
     */
    String getIndexFieldUrl();

    /**
     * Get the value for the key 'index.field.doc_id'. <br>
     * The value is, e.g. doc_id <br>
     * comment: Field name for document ID in the index.