&lt;xs:element name="used" type="UsedType" minOccurs="0"/&gt;
    &lt;/xs:sequence&gt;
    &lt;xs:attribute name="tool"        type="xs:string"       use="required" /&gt;
    &lt;!-- format: YYYY-MM-DDThh:mm:ss[+|-hh:mm] timezone offset --&gt;
    &lt;xs:attribute name="timestamp"   type="xs:dateTime"     use="required" /&gt;
  &lt;/xs:complexType&gt;

  &lt;!-- repository types --&gt;
  &lt;xs:simpleType name="RepositoryTypeEnum"&gt;
    &lt;xs:restriction base="xs:string"&gt;
      &lt;xs:enumeration valu