P5: Guidelines for Electronic Text Encoding and Interchange

<rng:element name="moduleRef">
<rng:ref name="att.global.attributes"/>
<rng:choice>
 <rng:optional>
  <rng:attribute name="key">
   <rng:data type="NCName"/>
  </rng:attribute>
 </rng:optional>
 <rng:optional>
  <rng:attribute name="url">
   <rng:ref name="data.pointer"/>
  </rng:attribute>
 </rng:optional>
</rng:choice>
<rng:optional>
 <rng:ref name="content"/>
</rng:optional>
</rng:element>

element moduleRef
{
   att.global.attributes,
   ( attribute key { xsd:NCName }? | attribute url { data.pointer }? ),
   content?
}