It's important to note that processors are allowed, if they wish, simply to ignore the comments in a file. This means that if you're building an XML application, you should never rely on anything that shows up in a comment (this sleazy trick is far too common in HTML). If you need to put material in a document that is not part of a document, but is available to programs that read it, that's what Processing Instructions, described in the next section, are for.
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