30114-2: ignorables, app-defined ext elems, or custom OPC parts

[MURATA Makoto]

Summary: Avoid ignorable elements but use application-defined
extension elements or additional OPC parts instead.

I have thought that we should use ignorable elements for representing
data for character repertoire checking.  The first CD does use them.
This approach is easy to standardize and implement, but existing
implementations will surely throw away data for character repertoire
checking.  I have stupidly thought that this is OK.  But no users are
willing to use what is thrown away by existing MS Office and Libre
Office.

The use of application-defined extension elements ensures that data
for character repertoire checking is preserved.  But locations of
application-defined extension elements are already set in stone, and
cannot be changed without making existing implementations
non-conformant.

Another option is to use additional OPC parts.  If implementations do
not throw away targets of valid relationships, data for character
repertoire checking is preserved.

I can imagine that the use of additional OPC parts has its own
problems.  But we should try seriously.

Regards,
Makoto
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