Restricting values in RNG for db.audience.attribute

We are profiling on db.audience.attribute and would like to enforce values to be within a predefined enumeration.

The following is working but is too restrictive: 

<define name="db.audience.attribute">

<attribute name="audience">

<ref name="my.audience" />

</attribute></define>

<define name="my.audience">

<choice>

<value>gui</value>

<value>admin</value>

...

</choice></define>

The problem is that our singlesourced and highly modular DocBook XML files use semicolon separated lists of multiple audience values. And validation will fail because the above shema only allows a single value from my.audience. 

So I tried to add the following and ref my.audience.list in stead of my.audience from the attribute definition:

<define name="my.audience.list">

<ref name="my.audience" />

<zeroOrMore>

<value>;</value>

<ref name="my.audience" />

</zeroOrMore>

</define>

At this point I get validation errors like "group of string or data element". How do I tweak the docbookxi.rng (in my customization layer rng file) to allow this construction with semicolons, which is clearly allowed by the shema without enforcing enumeration values?

Bergfrid Skaara