XPath positions

[online.identity.9]

I'm trying to define an xpath expression for conditional formatting of
text boxes. What I'd like to do is compare the value of the node to
the value of another node and highlight the box if the values are
different. Simple right? Here's the problem.
I've got a schema like this
<blah>
<values>
<value></value>
<value></value>
<value></value>
</values>
<previous>
<values>
<value></value>
<value></value>
<value></value>
</values>
</previous>
</blah>

The number of value tags is dynamic, what I'd like to do is compare "/
blah/values/value[x]" to /blah/previous/values/value[x]". Initially,
i tried something like ". != /blah/previous/values/value[position() =
count(preceding-sibling::*) + 1] , but of course that didn't work.

Any ideas?