[xsl] Problem matching part of a string

["Kent Seegmiller" <hookjaw20@xxxxxxxxxxx>]

OK here is my problem:
I have 2 source docs.  Doc 'A' has this:
<vendors>
 <vendor>DAN MORRISON MEAT PIES</vendor>
 <vendor>BOISE CASCADE</vendor>
 <vendor>CANDY TECH INC</vendor>
 <vendor>WF</vendor>
</vendors>

And Doc 'B' has this:

...
<vendor name="..."/>
<vendor name="..."/>
<vendor name="..."/>
<vendor name="..."/>
...

And I want to create a third xml doc using xslt2.0 that cross references the 
first part of @name of doc 'B' with the vendor element in doc 'A' and if so, 
make the claimable att. 'true'. There are some 1300 vendor names in doc 'B' 
and there are some 100 vendors that begin with 'WF'. So I get this in the 
third xml.

...
<vendor name="..." claimable="true/false"/>
<vendor name="..." claimable="true/false"/>
<vendor name="..." claimable="true/false"/>
<vendor name="..." claimable="true/false"/>
...

but I am having problems with the substring function.

-Thanks, KS