Including external XML file for example code

I am trying to include an XML document that is an example in my docbook 
article. I have been around, but could not get what I need. Google did 
return the pages I wanted. Maybe I am using the wrong key words. Can 
anyone suggest a link for a tutorial or tell me how to get this done. 
The example XML document is a full XML document and is part of a program 
so, there can be DOCTYPE and xml header.