[xml] Schematron support?

[Michael Ludwig <mlu as-guides com>]

I just tried out the Schematron support in LibXML2.

# cat test.sch
<schema xmlns="http://purl.oclc.org/dsdl/schematron";>
  <title>Test</title>
  <ns prefix="asg" uri="http://as-guides.com/wev1"/>
  <pattern>
    <rule context="asg:Broadcast[ tip]">
      <assert test="asg:Image">Wir haben ein Bild.</assert>
    </rule>
  </pattern>
</schema>

# xmllint --version
xmllint: using libxml version 20702
   compiled with: [...] Schemas Schematron [...]

# xmllint --noout --schematron test.sch /tmp/t1.xml
/tmp/t1.xml validation generated an internal error

The doc t1.xml is alright; its document element matches
the Schematron <pattern>. What does the error message mean?

Michael Ludwig