Revised "MCE semantics"

[MURATA Makoto]

> In X.4 (ExtensionElements), the current ext elements in Part 1/Part 4 are listed.  I think what we actually want here is the extLst elements.  Those are the OPC extension elements and ext is just a regular Part 1/Part 4 element that goes inside it.  This is the list of extLst I come up with:
> - http://purl.oclc.org/ooxml/spreadsheetml/main
> - http://purl.oclc.org/ooxml/presentationml/main
> - http://purl.oclc.org/ooxml/drawingml/main
>  - http://purl.oclc.org/ooxml/drawingml/chart

If extLst elements are app-defined ext elems,

<extLst>
  <foo xmlns="http://www.example.org/1"/>
  <ext>....</ext>
</extLst>

will be untouched and OOXML applications will receive this
foo element.  But dml-main.xsd (and dml-main.rnc) does
not allow such foreign elements.

Regards,
Makoto