Question about attribute match expressions

[awesomenode]

Hi!

I'm trying attribute match expressions, like mentioned in RFC6241.

When I try something similar, I get the following error from netopeer though:

<rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="">
  <get>
    <filter type="subtree">
      <t:c1 xmlns:t="urn:example:test">
        <t:l1 t:v1="A"/>
      </t:c1>
    </filter>
  </get>
</rpc>

<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-
id="9">
  <rpc-error>
    <error-type>application</error-type>
    <error-tag>operation-failed</error-tag>
    <error-severity>error</error-severity>
    <error-message xml:lang="en">Cannot filter based on unknown attributes.</error-message>
  </rpc-error>
</rpc-reply>

Is it possible to use attribute match expressions the way mentioned in the RFC, or am I doing something wrong?

I use the latest devel libyang, sysrepo, netopeer2 versions. I attached the YANG file for reference.

test.zip

Thanks!