<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0" 
         datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
  <start>
    <optional>
      <ref name="element-tags"/>
    </optional>
  </start>

  <define name="element-tags">
    <element name="tags">
      <zeroOrMore>
        <element name="tag">
          <attribute name="id"><data type="ID"/></attribute>
          <oneOrMore>
            <element name="obj_ref">
              <attribute name="id "><data type="IDREF"/></attribute>
            </element>
          </oneOrMore>
        </element>
      </zeroOrMore>
    </element>
  </define>

</grammar>