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The linking abilities of XML systems are much more powerful than those of HTML, so you'll be able to do much more with them. Existing HREF-style links will remain usable, but the new linking technology is based on the lessons learned in the development of other standards involving hypertext, such as TEI and HyTime, which let you manage bidirectional and multi-way links, as well as links to a span of text (within your own or other documents) rather than to a single point. These features have been available to SGML users for many years, so there is considerable experience and expertise available in using them.
The XML Linking Specification (XLink) and XML Extended Pointer Specification (XPointer) documents contain a detailed draft specification. An XML link can be either a URL or a TEI-style Extended Pointer (XPointer), or both. A URL on its own is assumed to be a resource; if an XPointer or XLink follows it, it is assumed to be a sub-resource of that URL; an XPointer on its own is assumed to apply to the current document (all exactly as with HTML).
An XLink is always preceded by one of #, ?, or |. The # and ? mean the same as in HTML applications; the | means the sub-resource can be found by applying the link to the resource, but the method of doing this is left to the application. An XPointer can only follow a #.
The TEI Extended Pointer Notation (EPN) is much more powerful than the fragment address on the end of some URLs, as it allows you to specify the location of a link end using the structure of the document as well as (or in addition to) known, fixed points like IDs. For example, the linked second occurrence of the word `XPointer' two paragraphs back could be referred to as (hypertext).child(2,*).child(2,#element,'p').child(3,#element,'link'), meaning the third link element within the second paragraph within the second object in the element whose ID is hypertext (this question). Count the objects from the start of this question in the XML source (which has the ID hypertext):
the first child object is the title of the question (
);
the second child object is the answer (the element);
within the element go to the second paragraph;
count to the third link.
David Megginson has produced an xpointer function for Emacs/psgml which will deduce an XPointer for any location in an XML document.
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