custom xsl sorting

Hi,

I have to perform xsl sorting based on time period
instead of regular ascii or integer based sorting. For
ex: According to ascii, BYZANTINE  comes before MIDDLE
BRONZE. But, according to this sort(time period):
MIDDLE BRONZE comes before BYZANTINE. 

My initial idea is like this. A separate xml:
'period.xml' has this period to ascii mapping.
<entry period="middlebronze">1</entry>
<entry period="byzantine">2</entry>
<entry period="Roman">3</entry>
<entry period="islamic">4</entry>
......
----------------------------------------
Now, my stylesheet should be applied to a dynamic xml
page & should order period values occur in that page.

<dynamicpage>
<val>Roman</val>
<val>Islamic</val>
<val> byzantine </val>
</dynamicpage>

desired output after xslt application:

byzantine,  Roman, Islamic

Please help regarding how xslt should look like?

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