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How do you go about including processing instructions?

Processing instructions are simply snippets of code that instruct the program accessing your document on where to go to find additional information needed to process the document.  You’ve been doing this already… every time you declare a stylesheet, as in the example of:

            <?xml-stylesheet type=”text/css” href=”cats.css”?>

you’re using processing instructions.  When the browser or program reaches that line, the XML document is basically telling it, ”Hey!  There’s more information that you need in order to do this right… you need a stylesheet.  It’s a CSS file, written in text, and it’s located over there at cats.css.”

Look back through some of the examples that you’ve seen and some of the work that you’ve done.  See the processing instructions?  Even when you’re referencing external DTD’s you use processing instructions.  All that they do is send the program somewhere else, so that it can gather the information that it needs and then come back.