public int getSubIntervalCount(Comparable rowKey, Comparable columnKey) {
		int r = getRowIndex(rowKey);
		int c = getColumnIndex(columnKey);
		if (c != -1) {
            return this.underlying.getSubIntervalCount(r,
            		c + this.firstCategoryIndex);
		}
		else {
			throw new UnknownKeyException("Unknown columnKey: " + columnKey);