* @return The base value for the extent calculation.
*/
private int computeBase(final FoContext context, final FObj fobj) {
final DtWritingMode writingMode = fobj.getPropertyList().getWritingMode(
fobj, context);
AbsoluteAxis absoluteAxis = null;
if (fobj instanceof RegionBefore || fobj instanceof RegionAfter) {
absoluteAxis = writingMode.getAbsoluteAxis(
RelativeAxis.BLOCK_PROGRESSION);
} else if (fobj instanceof RegionStart || fobj instanceof RegionEnd) {
absoluteAxis = writingMode.getAbsoluteAxis(