Matches a list of words separated by white space
Each word has a type
Can have multiple types
Allow multiple years in the YEAR element:
<element name="YEAR">
  <list>
    <oneOrMore>
      <data type="gYear"/>
    </oneOrMore>
  </list>
</element>
element SONG {
  element TITLE     { text },
  element COMPOSER  { xsd:string }+,
  element PRODUCER  { xsd:string }*,
  element PUBLISHER { xsd:string }?,
  element LENGTH    { xsd:string }?,
  element YEAR {
    list { xsd:gYear+ }
  }?,
  element ARTIST    { xsd:string }+
}