[xml] filtering a resulting NodeSet with xPath?

[Agustín Villena <avillena tutopia com>]

Hi!
   I need to implement in a fast way the functionality of filter a nodeset 
(obtained applying an xPath to an XML), hopefully aplying a new xPath.
The xPath API only applies to and xmlDocPtr, not to and xmlNodeSetPtr...
The XMLs that i need to filter has as many as 100.000 nodes, therefore 
performence is a must!

Example

<root>
<elem id="0">1<elem>
<elem id="1">0<elem>
<elem id="0">0<elem>
</root>

First filter
/root/elem[ id="0"]

expected result:
<elem id="0">1<elem>
<elem id="0">0<elem>

Second filter
[.="1"]
<elem id="0">1<elem>


This is equivalent  to make an XSL that saves the first nodeset in a 
variable (named for example "result") and apply
exsl:node-set($result)[.="1"]

Any advice?
 Agustin