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CHAPTER 4 XPATH, XPOINTER, XINCLUDE, AND THE FUTURE 155 The function id() takes a string argument, which must be surrounded by quotes. This appears within an attribute whose value is enclosed in double quotes. For this reason, I used single quotes to encapsulate the string c1. When the attribute value is enclosed by single quotes, the string needs to be encapsulated by double quotes, like xpointer=’xpointer(id(”c1″))’. This returns the courseelement identified by the ID c1from the courses.xmldocument. When included, the resulting document looks like this: Introduction to Languages Including documents, text, and fragments is a straightforward and simple process. All that is required is the addition of an include element, which resides in the http://www.w3.org/ 2001/XInclude namespace, and the location of the resource to be included. Within this section you have come to know this as the xi:include element. XInclude also offers a form of error handling, which is covered next, in the event you encounter a problem with the xinclude. xi:fallback Sometimes an XInclude may fail. It could be because of a problem accessing the remote resource or a possibly invalid selection of data. Normally this would cause an error in processing. XInclude offers a way to handle this and use other functionality in the event of an error. You do this using the xi:fallback element. The fallback element is referenced here using xi:fallback. It falls under the same rules as the xi:includeelement in respect to the namespace. It must reside within the http://www.w3.org/ 2001/XIncludenamespace, which for this chapter has been associated with the xiprefix. This element lives as a child of the xi:includeelement and has no attributes. When an error occurs from the xi:includeelement, the contents of the xi:fallbackelement are used for replacement. Sometimes a network may be unavailable, or an Internet connection goes down. It is also possible that the filename of the remote resource was mistyped in the xi:xincludehref attribute. Each of these would cause the include to fail. Take the case of an invalid href: This href is pointing to coursesBAD.xml, which is a file that does not exist. Processing the xinclude will result in at a minimum a parser warning and possibly an unrecoverable parser

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