Ruby-XCAPClient

http://tools.ietf.org/html/rfc4825#section-7.10
http://tools.ietf.org/html/rfc4825#section-10

An example:

NS = { "ccpp" => "urn:ietf:params:xml:ns:common-policy" }

@client.get_element("pres-rules", nil,
  '/ccpp:ruleset/ccpp:rule[@id="pres_whitelist"]/ccpp:conditions/ccpp:identity/namespace::*',
  NS, check_etag=false)

It generates a request like:

GET
/xcap-root/pres-rules/users/xcaptest@oversip.net/pres-rules/~~/ccpp:ruleset/ccpp:rule%5b@id=%22pres_whitelist%22%5d/ccpp:conditions/ccpp:identity/namespace::*
?xmlns(ccpp=urn:ietf:params:xml:ns:common-policy)
HTTP/1.1

And it should retrieve from the server a reply like:

HTTP/1.1 200 OK
Content-Type: application/xcap-ns+xml

<identity xmlns:pr="urn:ietf:params:xml:ns:pres-rules" 
  xmlns:cp="urn:ietf:params:xml:ns:common-policy" />

Unfortunatelly OpenXCAP doesn't implement such a query: http://openxcap.org/ticket/124