using a node-set as a predicate expression

Hi

I recently used a variable referencing a node-set (x) as a predicate
expression. I was expecting that the initial node-set (y), to which the
predicate was applied, would be filtered as if each node from y was
being compared to the node-set x. What I think I observed was that y was
being compared to x as a complete node-set.

To be more explicit, I was applying the preceding-sibling axis:

For the following node-set

      <xsl:variable name="subnodes"
select="allnodes['simple-predicate']"/>

I applied two strategies using a given node from "subnodes" as the
context node:

(a) simply add "subnodes" as the predicate

      <xsl:variable name="preceding"
select="preceding-sibbling::mynodetest[$subnodes]"/>

(b)explicitly define a node by node-set comparison

      <xsl:variable name="preceding"
select="preceding-sibbling::mynodetest[self::* = $subnodes]"/> 

I expected the results from (a) and (b) to be identical.

Can someone please explain?