syntax for domain usage declaration

I think it is correct as far as it goes. It is hard to read because of
the different usages of paren.

It doesn't fully describe the domains attribute value since attribute
domain declarations take the form:

a(module1 moduleN) where module1 is always either "props" or "base".

I don't think we have any real attribute domain declarations anywhere in
the DITA DTDs or XSDs. That is too bad since it makes finding a real
example to look at hard.

-Jeff

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