gio/tests/schema-tests/extend-and-shadow.gschema.xml

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

  <schema id='b' extends='a'>
    <key name='bar' type='s'>
      <default>''</default>
    </key>
  </schema>

  <schema id='c' extends='a'>
    <key name='foo' type='s'/>
  </schema>
</schemalist>