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DTD Entity

Since it isn't quite dead yet (just mortally wouned), I have a question DTD Elements.

Note: Beginner's question is being posed. Please excuse any cases of gross lack of knowledge.

We are working on DTD with the following requirement:

- Base element consists of Child elements First, Second, Third, and Fourth.
- First must appear under Base.
- Second, Third, and Fourth may appear zero or one time.
- Order is not important for elements Second, Third, and Fourth.

The original statement in my DTD was

<!ELEMENT Base (First, Second?, Third?, Fourth?)>

Which imposes an order constraint on the elements. It was suggested that

<!ELEMENT Base (First (Second|Third|Fourth)?)>

was more correct. However, this appears it will allow only Second, Third, _or_ Fourth element, not all three. Is 
this correct?


<!ELEMENT Base (First (Second|Third|Fourth)*)>

seems to allow enough elements (zero or one of each) but also allows more than one.

Is there a good way to address this situation or is this a limit of DTDs? Do Schemas handle this any better?

Again, thank you for you patience.

	- Richard


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