Combined proposal for header extension mechanism

["Adams, Robert" <robert.adams@intel.com>]

Both Chris and I have made suggestions for making the header extensible.
Here is a concrete strawman to work over.  I tagged the individual points so
they can be discussed separately.

HE1a: optional (occur 0..1) <extension> element added to the end of both
<ebXMLHeader> and <DocumentReference>
HE2: <extension> has one attribute of "MustUnderstand" who's value is either
"0" or "1"
HE3: the "MustUnderstand" attribute on <extension> is optional and has a
default value of "1" (true)
HE4: the contents of <extension> must be namespace qualified
HE5: if the receiver does not know the namespace specified and
"MustUnderstand" is "1" (true), an error occurs

An alternative is for the whole package extension to be in the Manifest
after the DocumentReference's
HE1b: optional (occur 0..1) <extension> element added to both <Manifest> and
<DocumentReference>

DTDs and schema's to follow.

	Robert Adams
	mailto:Robert.Adams@intel.com
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