John replies to Ron: Differences

[John Cowan]

Ron Bourret wrote:

> Content models of PCDATA and EMPTY represent the terminals in DTDs.

True, but not all data models have finite expansion.  HTML
does not, e.g.

> That is, bold, italic, etc. have a PCDATA content model.

Not typically.  More likely, each would have a mixed model
so that we can handle italic within bold and vice versa.
7A separate BI element could do that job otherwise, but
that leads to a combinatorial explosion when there are
six or seven appearance tags.

> Furthermore, in more data (as
> opposed to document) applications of XML, PCDATA-only content seems to be
> relatively common.

Perhaps true.  But maybe that's a historical artifact due to there
being no standard way to express rich text within databases.

> For the moment, I have left PCData in and required Mixed to
> take at least one ElementRef, per your earlier suggestion.

Okay.  BTW, it occurred to me today that SEQ and CHOICE (using
my terminology) should be compelled to have at least two children,
with content models

<!ELEMENT SEQ ((CHOICE | REF), (CHOICE | REF)+)>
<!ELEMENT CHOICE ((SEQ | REF), (SEQ | REF)+)>

Otherwise, we have

	<CHOICE><REF name="foo"/></CHOICE>
	<SEQ><REF name="foo"/></SEQ>
	<REF name="foo"/>

all with the same meaning.

-- 
John Cowan	http://www.ccil.org/~cowan		cowan at ccil.org
	You tollerday donsk?  N.  You tolkatiff scowegian?  Nn.
	You spigotty anglease?  Nnn.  You phonio saxo?  Nnnn.
		Clear all so!  'Tis a Jute.... (Finnegans Wake 16.5)

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