mei:mei_att.source.attribute.source

<xs:attributeGroup name="mei_att.source.attribute.source">
  <xs:attribute name="source">
    <xs:annotation>
      <xs:documentation>Contains a list of one or more pointers indicating the sources which attest to a given reading. Each value should correspond to the ID of a <source> element located in the document header.</xs:documentation>
    </xs:annotation>
    <xs:simpleType>
      <xs:restriction>
        <xs:simpleType>
          <xs:list itemType="mei:mei_data.URI"/>
        </xs:simpleType>
        <xs:minLength value="1"/>
      </xs:restriction>
    </xs:simpleType>
  </xs:attribute>
</xs:attributeGroup>