XML1..1: Can we please have a derived atomic type?

One of the major things missing from version 1 is the ability to define new 
atomic types as a combination of existing types. As an example, the 
construction of a "currency" type that might consist of the two simple 
atomic types:

eg. currencycode: string, restricted to "£", "$" etc.
     currencyval : decimal, 2 fractional digits

->  currency    : union of: currencycode & currencyval

So, this new atomic type would permit values such as £27.20 and $7.01 but 
would reject both "£" and "15.88" and "15.1" etc.

Comments would be welcome.