xpath+schematron rule (is possible...)

Greetings,

I wonder if is possible to verify with XPath the following "pseudo" rule
for schematron:

-I want to match (assert) an element <a> with an attribute @match='true'
that its @index value will exist in another element <b> with the same
attribute @index.

For instance,

<root>
   <list1>
   	<a index="1" match="true">
 	<a index="2" match="false">
	<a index="3" match="true">
   </list1>
   <list2>
   	<b index="1">
 	<b index="2">
   </list2>
</root>

Now, I want to pick a rule with XPath and schematron which will detect
the non existence of <b> with @index='3'.


I tried this rule but it's malformed:

<rule context="root" >
	<assert test="a[@match='true']@index=b@index"> 
	fail </assert>
</rule>

Any help would be very appreciated.

regards,
Pedro Salazar.
-- 
PS
pedro-b-salazar_at_ptinovacao_dot_pt
PGP:0E129E31D803BC61