Hygenic MCE using Schematron

In recent blogs I have been looking at aspects of extensions: the Markup Compatibility and Extension (MCE) standard and the kinds of constraints needed in order to allow documents that have extensions but which can play everywhere: Minimal Open Documents Using Standards: MODUS.

Here is a Schematron schema for the kinds of constraints I am suggesting would be appropriate:

<sch:pattern>

  <sch:title>MCE elements must allow one standard alternative</sch:title>


  <sch:rule context="mce:alternativeContent">

     <sch:assert test="mce:choice[not(@mustUnderstand)][*] 

         or mce:fallback[not(@mustUnderstand)][*]">

     Every alternative content section must have at least

     one choice (or fallback) which is not marked by any

     mustUnderstand attribute, and which is not empty.

     </sch:assert>

    <sch:assert test="not( @mustUnderstand)">

    An alternative content element must have at least one

    choice that does not require any extension namespaces.

    </sch:assert>

  </sch:rule>

  <sch:rule context="mce:choice[not(@mustUnderstand)]//* 

     | mce:fallback[not(@mustUnderstand)]//*">

     <sch:assert test="namespace() = 'allowed-ns-1' 

            or namespace() = 'allowed-ns-2'  ... ">

     A choice or fallback that has no mustUnderstand attribute

     must only contain known elements at any level. 

     </sch:assert>

  </sch:rule>

</sch:pattern>