unknown and foreign elements

[Michael Priestley <mpriestl@ca.ibm.com>]

Belatedly providing my reasoning for
making the foreign element a specialization of a more general unknown element:

- "unknown" for me expresses
that the purpose of the content is unknown; and so default behavior will
be to ignore the content when outputting the topic. Some examples of elements
that could be specialized from unknown could include required-cleanup and
reusable-elements.
- "foreign" is a particular
kind of content with an unknown purpose, in which we at least know why
it's unknown (it's a non-DITA vocabulary). Some examples could be mathml,
SVG...

I hope this clarifies,

Michael Priestley
IBM DITA Architect and Classification Schema PDT Lead
mpriestl@ca.ibm.com
http://dita.xml.org/blog/25