<xs:complexType name="backup">
  <xs:annotation>
    <xs:documentation>The backup and forward elements are required to coordinate multiple voices in one part, including music on multiple staves. The backup type is generally used to move between voices and staves. Thus the backup element does not include voice or staff elements. Duration values should always be positive, and should not cross measure boundaries.</xs:documentation>
  </xs:annotation>
  <xs:sequence>
    <xs:group ref="duration"/>
    <xs:group ref="editorial"/>
  </xs:sequence>
</xs:complexType>