Article in Set not Allowed: Why?

Hi,

according to the TDG[1], set allows another set or book, but no
article. I'm wondering if there is any reason for this. Just curiosity. :)

It would probably convenient to allow articles in a set. Currently, a
writer is forced to add articles into a book, although they may not fit
thematically together.

What do you think?

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[1] http://www.docbook.org/tdg/en/html/set.html

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Gruß/Regards,
    Thomas Schraitle