Foreign Element

["Paul Prescod" <paul.prescod@blastradius.com>]

We did not get around to discussing the foreign element on the list. It
seemed that Michael felt that foreign is the appropriate base for any
element that has a content model of any. It seemed to me that an element
holding unknown non-DITA content is quite different than one holding
arbitrary DITA content. I don't see a strong argument for unifying those
in a single element other than the fact that they happen to have a
shared content model. The class hierarchy that makes sense to me looks
like this.

topic/Unknown
	mathml/MathML
	svg/SVG
	...

???/AnyDitaContent
	component/ReusableContent
	topic/RequiredCleanup
	...

The semantics of "Unknown" are to try and find a processor capable of
handling the unknown data. If the processor cannot be found or cannot
deal with the data, then alternative text should be published.
Generalizing MathML to Unknown is valuable because of this handling.

The semantics of "AnyDitaContent" are to supress unless otherwise
instructed. There is actually not a ton of value in the common behaviour
of the "AnyDitaContent" specializations but it feels neater to unify
them. Note that it is difficult to envision how to make RequiredCleanup
a specialization of anything else because it is already in the "topic"
module.

 Paul Prescod