Extending type definitions using RelaxNG Compact

Hi,

We are using xsd:string as the base datatype for our types: e.g.

  datatypes xsd = "http://www.w3.org/2001/XMLSchema-datatypes"

  FirstName = xsd:string {
      maxLength = "30"
  }

  LastName = xsd:string {
      maxLength = "60"
  }

We'd like to introduce a more restricted string type, something like:

  AlphaString = xsd:string {
      pattern = "[a-zA-Z]*"
  }

  FirstName = AlphaString {
      maxLength = "30"
  }

  LastName = AlphaString {
      maxLength = "60"
  }

which would be equivalent to:

  FirstName = xsd:string {
      pattern = "[a-zA-Z]*"
      maxLength = "30"
  }

  LastName = xsd:string {
      pattern = "[a-zA-Z]*"
      maxLength = "60"
  }

Unfortunately, the "FirstName = AlphaString" example doesn't work.  Does
anyone know a way of achieving what I'm trying to do?  Is it even
possible with RelaxNG?

Cheers
Kris.