Co-Constraints (DTD)

DTDs don’t support co-constraints:

<!ENTITY biblio.class.attribute "
        class   (doi|isbn|issn|libraryofcongress
                 |pubnumber|uri|other) #IMPLIED
        otherclass CDATA        #IMPLIED
">

The desired semantic is:

  • If class is “other”, then otherclass must be specified, otherwise

  • The otherclass must not be specified.