RELAX NG validation - how to allow a universal attribute

Question

I'm designing my XML so that every element in it may specify some universal attribute. For example, say the initial XML was:

<RootElement>
    <Sub>
        ...
    </Sub>

    <Single1/>

    <Single2/>

</RootElement>

Then the following is also valid:

<RootElement UniversalAttr="true">
    <Sub UniversalAttr="false">
        ...
    </Sub>

    <Single1/>

    <Single2 UniversalAttr="true"/>
</RootElement>

I need this ability for being able to later filter the XML at runtime, based on the attribute values.

The only method I know is to specify in the schema all the possible places the attribute may appear. So that the following block would appear many many times in the schema:

<optional>
    <attribute name="UniversalAttr">
        <choice>
            <value>true</value>
            <value>false</value>
        </choice>
    </attribute>
</optional>

But I'm looking for a method where I could specify once that this attribute may appear for any element the XML author wishes.

Can this be done?

EDIT: a second best solution would be to be able to define an "ignored" attribute.

Answer 1

my adhoc idea: you can define your universal attribute like

  <define name="att.universal">
      <optional>
         <attribute name="UniversalAttr">
           <choice>
             <value>true</value>
             <value>false</value>
           </choice>
         </attribute>
      </optional>
   </define>

The elements that need the universal attribute reference it

 <element name="xyz">
     <ref name="att.universal"/>
     ....
 </element>