TEI: Guidelines for Electronic Text Encoding and Interchange

<unitDecl>
<unitDef xml:id="pechys" type="length">
<label>πῆχυς</label>
<placeName ref="#athens"/>
<conversion fromUnit="#daktylos"
toUnit="#pechys" formula="$fromUnit div 24"/>
<desc>Equivalent to a cubit or 24 daktyloi.</desc>
</unitDef>
<unitDef xml:id="daktylos" type="length">
<label>δάκτυλος</label>
<placeName ref="#athens"/>
<desc>A basic unit of length equivalent to one finger (or the size of a thumb) in ancient Greece.</desc>
</unitDef>
</unitDecl>

XML syntaxCompact syntax

<rng:element name="unitDef">
 <rng:ref name="att.global.attributes"/>
 <rng:ref name="att.global.analytic.attributes"/>
 <rng:ref name="att.global.change.attributes"/>
 <rng:ref name="att.global.facs.attributes"/>
 <rng:ref name="att.global.linking.attributes"/>
 <rng:ref name="att.global.rendition.attributes"/>
 <rng:ref name="att.global.responsibility.attributes"/>
 <rng:ref name="att.global.source.attributes"/>
 <rng:ref name="att.canonical.attributes"/>
 <rng:ref name="att.datable.attributes"/>
 <rng:ref name="att.datable.custom.attributes"/>
 <rng:ref name="att.datable.iso.attributes"/>
 <rng:ref name="att.datable.w3c.attributes"/>
 <rng:ref name="att.typed.attributes"/>
 <rng:oneOrMore>
 <rng:choice>
  <rng:ref name="model.labelLike"/>
  <rng:optional>
  <rng:ref name="model.placeNamePart"/>
  </rng:optional>
  <rng:optional>
  <rng:ref name="conversion"/>
  </rng:optional>
  <rng:optional>
  <rng:ref name="unit"/>
  </rng:optional>
 </rng:choice>
 </rng:oneOrMore>
</rng:element>

element unitDef
{
 att.global.attributes,
 att.global.analytic.attributes,
 att.global.change.attributes,
 att.global.facs.attributes,
 att.global.linking.attributes,
 att.global.rendition.attributes,
 att.global.responsibility.attributes,
 att.global.source.attributes,
 att.canonical.attributes,
 att.datable.attributes,
 att.datable.custom.attributes,
 att.datable.iso.attributes,
 att.datable.w3c.attributes,
 att.typed.attributes,
 ( model.labelLike | model.placeNamePart? | conversion? | unit? )+
}