P5: Guidelines for Electronic Text Encoding and Interchange

<rng:element name="tree">
<rng:ref name="att.global.attributes"/>
<rng:optional>
 <rng:attribute name="arity">
  <rng:ref name="data.count"/>
 </rng:attribute>
</rng:optional>
<rng:attribute name="ord" a:defaultValue="true">
 <rng:choice>
  <rng:value>true</rng:value>
  <rng:value>partial</rng:value>
  <rng:value>false</rng:value>
 </rng:choice>
</rng:attribute>
<rng:optional>
 <rng:attribute name="order">
  <rng:ref name="data.count"/>
 </rng:attribute>
</rng:optional>
<rng:group>
 <rng:optional>
  <rng:ref name="label"/>
 </rng:optional>
 <rng:group>
  <rng:zeroOrMore>
   <rng:choice>
    <rng:ref name="leaf"/>
    <rng:ref name="iNode"/>
   </rng:choice>
  </rng:zeroOrMore>
  <rng:ref name="root"/>
  <rng:zeroOrMore>
   <rng:choice>
    <rng:ref name="leaf"/>
    <rng:ref name="iNode"/>
   </rng:choice>
  </rng:zeroOrMore>
 </rng:group>
</rng:group>
</rng:element>

element tree
{
   att.global.attributes,
   attribute arity { data.count }?,
   attribute ord { "true" | "partial" | "false" },
   attribute order { data.count }?,
   ( label?, ( ( leaf | iNode )*, root, ( leaf | iNode )* ) )
}