P5: Guidelines for Electronic Text Encoding and Interchange

<rng:element name="forest">
<rng:ref name="att.global.attributes"/>
<rng:optional>
 <rng:attribute name="type">
  <rng:ref name="data.enumerated"/>
 </rng:attribute>
</rng:optional>
<rng:oneOrMore>
 <rng:choice>
  <rng:ref name="tree"/>
  <rng:ref name="eTree"/>
  <rng:ref name="triangle"/>
 </rng:choice>
</rng:oneOrMore>
</rng:element>

element forest
{
   att.global.attributes,
   attribute type { data.enumerated }?,
   ( tree | eTree | triangle )+
}