schemas: can I restrict an elem's content based on att value?

[Imran Rashid <imranr@wolfram.com>]

sorry, another schema newbie question.

I've been scanning the Schema CR's, but I can't seem to find anythig
relating to this.
    Can I restrict an element's content, based on one of its attributes?

for example:
say, <test/> could take numbers or strings.
could I have a "type" attribute, that coudl be either number or string, and
the contents type would have to be a numer or string respectively?

good:
<test type="number">5</test>
bad:
<test type="number">hello</test>


thanks,
Imran Rashid