From Daniel Debrunner:
The XML operators and data type include text about requiring Xalan etc. When trying out XML I felt lost at this point because there was no information on what I was meant to do. It was the feeling I was being told "You need to do something with Xalan to get this to work, but you are going to have to figure that out yourself". It stopped me from evaluating Mustang with XML. Can there be information in the developers' guide about this, and a link to that information from these Usage notes?