how to add this <fo:layout-master-set>
<fo:simple-page-master master-name="simpleA5" page-height="210mm" page-width="148mm" margin-top="5mm" margin-bottom="5mm" margin-left="5mm" margin-right="5mm">
<fo:region-body margin-top="20mm"/>
<fo:region-before extent="20mm"/>
</fo:simple-page-master>
</fo:layout-master-set> in an xml str