[xml] validation fails for embedded RELAX NG schema

[Jerzy Jalocha N <jjalocha gmail com>]

Hi everyone, this is my first post here. Thanks for the amazing
xsltproc and xmllint tools!

I would like to use a RELAX NG schema that is embedded inside some
"outer" XML file:

<?xml version='1.0' encoding='UTF-8'?>
<wrapper xmlns='http://namespace.net/something'>
   <!-- Here would go a lot of stuff that belongs to the wrapper. -->
   <grammar xmlns='http://relaxng.org/ns/structure/1.0'>
      <start>
         <element name='data'>
            <zeroOrMore>
               <element name='entry'>
                  <text/>
               </element>
            </zeroOrMore>
         </element>
      </start>
   </grammar>
</wrapper>

Sadly, xmllint (2.6.32) complains about an empty schema:

example-schema.xml:0: Relax-NG parser error : xmlRelaxNGParse:
example-schema.xml is empty
Relax-NG schema example-schema.xml failed to compile.

I know, that I could pre-process my "mixed" document, and extract the
schema with XSLT prior to validation. But this would make the whole
project quite a lot more complex, thus I'd like to avoid it, if
possible.

I guess, that I am not doing anything illegal by using two
vocabularies side-by-side, as long as I use separate namespaces, or am
I wrong?

Is this a known limitation of xmllint, or maybe a bug?

Any thoughts about this are appreciated.

Regards,
Jerzy