XML in a Nutshell, 3rd Edition

origin( )

The origin( ) function is useful when the document has been loaded from an out-of-line link. It refers to the node from which the user is initiating traversal, even if that is not the node that defines the link. For example, consider an extended link like this one. It has many novel elements, each of which is a locator that shares the same label:

<series xlink:type="extended" xmlns:xlink="http://www.w3.org/1999/xlink">
     
  <!-- locator elements -->
  <novel xlink:type="locator" xlink:label="oz"
         xlink:href="ftp://archive.org/pub/etext/etext93/wizoz10.txt">
    <title>The Wonderful Wizard of Oz</title>
    <year>1900</year>
  </novel>
  <novel xlink:type="locator" xlink:label="oz"
         xlink:href="ftp://archive.org/pub/etext/etext93/ozland10.txt">
    <title>The Marvelous Land of Oz</title>
    <year>1904</year>
  </novel>
  <novel xlink:type="locator" xlink:label="oz"
         xlink:href="ftp://archive.org/pub/etext/etext93/wizoz10.txt">
    <title>Ozma of Oz</title>
    <year>1907</year>
  </novel>
  <!-- many more novel elements... -->
     
  <sequel xlink:type="locator" xlink:label="next"
        xlink:href="#xpointer(origin( )/following-sibling::novel[1])" />
  <next xlink:type="arc" xlink:from="oz" xlink:to="next" />
     
</series>

The sequel element uses an XPointer and the origin( ) function to define a locator that points to the following novel in the series. If the user is reading The Wonderful Wizard of Oz, then the sequel element locates The Marvelous Land of Oz. If the user is reading The Marvelous Land of Oz, then that same sequel