gio/tests/schema-tests/override-then-key.gschema.xml

<schemalist>
  <schema id='base'>
    <key name='foo' type='s'>
      <default>'bar'</default>
    </key>
  </schema>

  <schema id='sub' extends='base'>
    <override name='foo'>'baz'</override>
  </schema>

  <schema id='sub2' extends='sub'>
    <key name='foo' type='s'/>
  </schema>
</schemalist>