unit

<xs:attribute name="unit" use="required">
  <xs:annotation>
    <xs:documentation>provides a conventional name for the kind of section changing at this milestone. Suggested values include: 1] page; 2] column; 3] line; 4] book; 5] poem; 6] canto; 7] speaker; 8] stanza; 9] act; 10] scene; 11] section; 12] absent; 13] unnumbered</xs:documentation>
  </xs:annotation>
  <xs:simpleType>
    <xs:union>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="page">
            <xs:annotation>
              <xs:documentation>physical page breaks (synonymous with the pb element).</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="column">
            <xs:annotation>
              <xs:documentation>column breaks.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="line">
            <xs:annotation>
              <xs:documentation>line breaks (synonymous with the lb element).</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="book">
            <xs:annotation>
              <xs:documentation>any units termed book, liber, etc.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="poem">
            <xs:annotation>
              <xs:documentation>individual poems in a collection.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="canto">
            <xs:annotation>
              <xs:documentation>cantos or other major sections of a poem.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="speaker">
            <xs:annotation>
              <xs:documentation>changes of speaker or narrator.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="stanza">
            <xs:annotation>
              <xs:documentation>stanzas within a poem, book, or canto.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="act">
            <xs:annotation>
              <xs:documentation>acts within a play.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="scene">
            <xs:annotation>
              <xs:documentation>scenes within a play or act.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="section">
            <xs:annotation>
              <xs:documentation>sections of any kind.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="absent">
            <xs:annotation>
              <xs:documentation>passages not present in the reference edition.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="unnumbered">
            <xs:annotation>
              <xs:documentation>passages present in the text, but not to be included as part of the reference.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:pattern value="(\p{L}|\p{N}|\p{P}|\p{S})+"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:union>
  </xs:simpleType>
</xs:attribute>