Request to relax PCDATA content model restriction

I've been corresponding with Terry Allen, who's doing an XMLish version of
DocBook.  DocBook has a lot of content models like the following:


where %xxx.char.mix; resolves to (#PCDATA|a|b|c...).  The current XML Lang
spec doesn't allow for this; production 45 is very specific about how many
pairs of parens can appear, rather than going for the generalized model
group solution of productions 40-44.

Of course, the outermost parens aren't really necessary; you can achieve
the desired effect with (%xxx.char.mix;)* instead of ((%xxx.char.mix;)*).
But where content models are heavily parameterized and you can't easily see
what kind of model you've got, this makes the DTD writer split hairs.

It should always be safe to throw another couple of paren pairs around a
model, and currently in XML you can't do this.  Can we consider loosening
this restriction?