String toStringDetail() {
        final StringBuilder sb = new StringBuilder(toString());
        for (int level = 0; level < MAX_TREE_DEPTH; level++) {
            final LevelCache lc = _levelCache[level];
            if (lc == null || lc._buffer == null) {
                break;
            } else {
                sb.append(Util.NEW_LINE);
                sb.append(level);
                sb.append(": ");
                sb.append(lc);
            }
        }
        return sb.toString();
    }