Placement of attribution in blockquotes

Here's something I've always wondered, and never got around to asking.
Why can't we have an attribution at the end of a blockquote?  One reason
that springs to mind is that we may have wanted to enforce a "no more
than one attribution per blockquote" rule, and I think that supporting
this option with DTDs would have allowed an attribution at the beginning
and the end of a blockquote.  This kind of constraint (attribution at
either the beginning or end but not both) would be enforceable using
Relax NG, though, would it not?

Take care,

    John L. Clark