suggestion: variables in schema

I cannot find this in schema 1.0, but I'm a novice so apoligies if it's there.

I'd like the simple ability to base a restriction on user data.

Ie something like:

<xs:element name="measurements">
   <xs:complexType>
     <xs:attribute name="threshold" type="positiveInteger">
       <xs:element name="length">
         <xs:simpleType>
           <xs:restriction base="xs:integer">
             <xs:minExclusive value="0"/>
             <xs:maxInclusive value="$threshold"/>
close tags etc..

This way the length data has to conform to the threshold restriction.

Thanks in advance,

Andrew