DDML - Xml format

DDML, aka Document Definition Markup Language, is a xml format created in 1999.
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Document Definition Markup Language (DDML) is an XML schema language proposed in 1999 by various contributors from the xml-dev electronic mailing list. It was published only as a W3C Note, not a Recommendation, and never found favor with developers. DDML began as XSchema, a reformulation of XML DTDs as full XML documents, so that elements and attributes, rather than declarations, could be used to describe a schema. Read more on Wikipedia...
DDML Wikipedia page
DDML first developed in http://www.xml.org/xml-dev/
Example from the web:
<?xml version="1.0"?>

<!DOCTYPE "DocumentDef">

<DocumentDef FileExtension="ddm" prefix="">

  <ElementDecl Name="DocumentDef" Root="Recommended">
    <Model>
      <Seq>
        <Ref Element="Doc" Frequency="Optional"/>
        <Ref Element="More" Frequency="Optional"/>
        <Choice Frequency="ZeroOrMore">
          <Ref Element="ElementDecl"/>
          <Ref Element="Model"/>
          <Ref Element="AttDef"/>
          <Ref Element="AttGroup"/>
          <Ref Element="Notation"/>
          <Ref Element="UnparsedEntity"/>
          <Ref Element="Enumeration"/>
          <Ref Element="DocumentDef"/>
        </Choice>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="xmlns" Required="Yes" AttValue="http://www.purl.org/NET/ddml/v1"/>
      <AttDef Name="DDML" prefix="xmlns" Required="Yes" AttValue="http://www.purl.org/NET/ddml/v1"/>
      <AttDef Name="ns"/>
      <AttDef Name="ElementNS"/>
      <AttDef Name="prefix" Type="Nmtoken"/>
      <AttDef Name="Version" Required="Yes" AttValue="1.0"/>
      <AttDef Name="MimeType" AttValue="application/xml"/>
      <AttDef Name="FileExtension" AttValue="xml"/>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="ElementDecl">
    <Model>
      <Seq>
        <Ref Element="Doc" Frequency="Optional"/>
        <Ref Element="More" Frequency="Optional"/>
        <Ref Element="Model"/>
        <Ref Element="AttGroup" Frequency="Optional"/>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="Name" Type="Nmtoken" Required="Yes"/>
      <AttDef Name="ns"/>
      <AttDef Name="prefix" Type="Nmtoken"/>
      <AttDef Name="id" Type="ID"/>
      <AttDef Name="Root" Type="Enumerated" AttValue="Possible">
        <Enumeration>
          <EnumerationValue Value="Recommended"/>
          <EnumerationValue Value="Possible"/>
          <EnumerationValue Value="Unlikely"/>
        </Enumeration>
      </AttDef>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Model">
    <Model>
      <Seq>
        <Ref Element="Doc" Frequency="Optional"/>
        <Ref Element="More" Frequency="Optional"/>
        <Choice>
          <Ref Element="Ref"/>
          <Ref Element="Choice"/>
          <Ref Element="Seq"/>
          <Ref Element="Empty"/>
          <Ref Element="Any"/>
          <Ref Element="PCData"/>
          <Ref Element="Mixed"/>
        </Choice>
      </Seq>    
    </Model>
    <AttGroup>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Empty">
    <Model>
      <Empty/>
    </Model>
    <AttGroup>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Any">
    <Model>
      <Empty/>
    </Model>
    <AttGroup>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="PCData">
    <Model>
      <Empty/>
    </Model>
    <AttGroup>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Ref">
    <Model>
      <Empty/>
    </Model>
    <AttGroup>
      <AttDef Name="Element" Type="Nmtoken" Required="Yes"/>
      <AttDef Name="ElementNS"/>
      <AttDef Name="id" Type="ID"/>
      <AttDef Name="Frequency" Type="Enumerated" AttValue="Required">
        <Enumeration>
          <EnumerationValue Value="Required"/>
          <EnumerationValue Value="Optional"/>
          <EnumerationValue Value="ZeroOrMore"/>
          <EnumerationValue Value="OneOrMore"/>
        </Enumeration>
      </AttDef>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Mixed">
    <Model>
      <Ref Element="Ref" Frequency="OneOrMore"/>
    </Model>
    <AttGroup>
      <AttDef Name="ElementNS"/>
      <AttDef Name="id" Type="ID"/>
      <AttDef Name="Frequency" Type="Enumerated" Required="Yes" AttValue="ZeroOrMore">
        <Enumeration>
          <EnumerationValue Value="ZeroOrMore"/>
        </Enumeration>
      </AttDef>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Choice">
    <Model>
      <Seq>
        <Choice>
          <Ref Element="Seq"/>
          <Ref Element="Ref"/>
          <Ref Element="Model"/>
        </Choice>
        <Choice Frequency="OneOrMore">
          <Ref Element="Seq"/>
          <Ref Element="Ref"/>
          <Ref Element="Model"/>
        </Choice>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="ElementNS"/>
      <AttDef Name="id" Type="ID"/>
      <AttDef Name="Frequency" Type="Enumerated" AttValue="Required">
        <Enumeration>
          <EnumerationValue Value="Required"/>
          <EnumerationValue Value="Optional"/>
          <EnumerationValue Value="ZeroOrMore"/>
          <EnumerationValue Value="OneOrMore"/>
        </Enumeration>
      </AttDef>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Seq">
    <Model>
      <Seq>
        <Choice>
          <Ref Element="Choice"/>
          <Ref Element="Ref"/>
          <Ref Element="Model"/>
        </Choice>
        <Choice Frequency="OneOrMore">
          <Ref Element="Choice"/>
          <Ref Element="Ref"/>
          <Ref Element="Model"/>
        </Choice>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="ElementNS"/>
      <AttDef Name="id" Type="ID"/>
      <AttDef Name="Frequency" Type="Enumerated" AttValue="Required">
        <Enumeration>
          <EnumerationValue Value="Required"/>
          <EnumerationValue Value="Optional"/>
          <EnumerationValue Value="ZeroOrMore"/>
          <EnumerationValue Value="OneOrMore"/>
        </Enumeration>
      </AttDef>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="AttGroup">
    <Model>
      <Seq>
        <Ref Element="Doc" Frequency="Optional"/>
        <Ref Element="More" Frequency="Optional"/>
        <Choice Frequency="ZeroOrMore">
           <Ref Element="AttDef"/>
           <Ref Element="AttGroup"/>
        </Choice>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="ns"/>
      <AttDef Name="prefix" Type="Nmtoken"/>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="AttDef">
    <Model>
      <Seq>
        <Ref Element="Doc" Frequency="Optional"/>
        <Ref Element="More" Frequency="Optional"/>
        <Ref Element="Enumeration" Frequency="Optional"/>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="Name" Type="Nmtoken" Required="Yes"/>
      <AttDef Name="ns"/>
      <AttDef Name="prefix" Type="Nmtoken"/>
      <AttDef Name="Type" Type="Enumerated" AttValue="CData">
        <Enumeration>
          <EnumerationValue Value="CData"/>
          <EnumerationValue Value="ID"/>
          <EnumerationValue Value="IDRef"/>
          <EnumerationValue Value="IDRefs"/>
          <EnumerationValue Value="Entity"/>
          <EnumerationValue Value="Entities"/>
          <EnumerationValue Value="Nmtoken"/>
          <EnumerationValue Value="Nmtokens"/>
          <EnumerationValue Value="Notation"/>
          <EnumerationValue Value="Enumerated"/>
        </Enumeration>
      </AttDef>
      <AttDef Name="Required" Type="Enumeration" AttValue="No">
        <Enumeration>
          <EnumerationValue Value="Yes"/>
          <EnumerationValue Value="No"/>
        </Enumeration>
      </AttDef>
      <AttDef Name="AttValue"/>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Enumeration">
    <Model>
      <Seq>
        <Ref Element="Doc" Frequen cy="Optional"/>
        <Ref Element="More" Frequency="Optional"/>
        <Ref Element="EnumerationValue" Frequency="OneOrMore"/>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="EnumerationValue">
    <Model>
      <Seq>
        <Ref Element="Doc" Frequency="Optional"/>
        <Ref Element="More" Frequency="Optional"/>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="Value" Required="Yes"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Notation">
    <Model>
      <Seq>
        <Ref Element="Doc" Frequency="Optional"/>
        <Ref Element="More" Frequency="Optional"/>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="Name" Type="Nmtoken" Required="Yes"/>
      <AttDef Name="PubidLiteral"/>
      <AttDef Name="SystemLiteral"/>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="UnparsedEntity">
    <Model>
      <Seq>
        <Ref Element="Doc" Frequency="Optional"/>
        <Ref Element="More" Frequency="Optional"/>
      </Seq>
    </Model>
    <AttGroup>
      <AttDef Name="Name" Type="Nmtoken" Required="Yes"/>
      <AttDef Name="SystemLiteral" Required="Yes"/>
      <AttDef Name="PubidLiteral"/>
      <AttDef Name="Notation" Type="Nmtoken" Required="Yes"/>
      <AttDef Name="id" Type="ID"/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="Doc" prefix="DDML">
    <Model>
       <!-- The struct model from IBTWSH goes here.
            Defining IBTWSH in DDML is left as an
            exercise to the reader.            ;       -->
    </Model>
    <AttGroup>
      <AttDef Name="xmlns" Required="Yes" AttValue=""/>
    </AttGroup>
  </ElementDecl>

  <ElementDecl Name="More" prefix="DDML">
    <Model>
      <Any/>
    </Model>
    <AttGroup>
      <AttDef Name="xmlns" AttValue=""/>
    </AttGroup>
  </ElementDecl>

</DocumentDef>