No, I didn't add that separate-file-feature for the xml files since I expect the xml files to be much smaller.

I tried to explain it in my original post already:

<config>
<area code="1" id="a"/>
<area code="2" id="a">
<area code="4" id="b"/>
<directory id="a">
...
</directory>
<directory id="b">
...
</directory>
</config>

The area codes are matched same way as in the .txt files. (The area codes are only used to find the directory to use, it is not removed from the number, unless the directory entry itself removes it with a <prefix> tag.)