Pointing to the XHTML schema

<?xml version="1.0"?>
<element name="SONG" xmlns="http://relaxng.org/ns/structure/1.0">
  <element name="TITLE">
    <text/>
  </element>
  <oneOrMore>
    <element name="COMPOSER">
      <text/>
    </element>
  </oneOrMore>
  <zeroOrMore>
    <element name="PRODUCER">
      <text/>
    </element>
  </zeroOrMore>
  <optional>
    <element name="PUBLISHER">
      <text/>
    </element>
  </optional>
  <optional>
    <element name="LENGTH">
      <text/>
    </element>
  </optional>
  <optional>
    <element name="YEAR">
      <text/>
    </element>
  </optional>
  <oneOrMore>
    <element name="ARTIST">
      <text/>
    </element>
  </oneOrMore>
  <optional>
    <element name="DESCRIPTION">
      <externalRef href="http://thaiopensource.com/relaxng/xhtml/xhtml-basic.rng"/>
    </element>
  </optional>
</element>

element SONG {
  element TITLE       { text },
  element COMPOSER    { text }+,
  element PRODUCER    { text }*,
  element PUBLISHER   { text }?,
  element LENGTH      { text }?,
  element YEAR        { text }?,
  element ARTIST      { text }+,
  element DESCRIPTION { external "xhtml-basic.rnc" }?
}