> in P4, yes. but surely in P5 the @value is restricted to a W3C
> datatype. So Art needs to peruse the W3C schema spec to see what
> they support.

Well, yes, currently value= of <num> is declared as xsd:decimal in
P5. But I submit it's not up to Art to go find out that W3C says that
our use of xsd:decimal means he needs to use 30000000000[1], but
rather it is up to us to ascertain whether or not there is user
demand for being able to represent really large numbers with a
different notation, and if so modify the declaration of value= of
<num> so that it's possible.

[1] See
    http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/#dt-decimal