[xml] xmlRegisterOutputCallbacks

[James Burgess <justrandombytes gmail com>]

Hi, 

  I want to use my own I/O routines to output XML using the xmlWriter API. xmlRegisterOutputCallbacks() looks 
like just the thing but I'm confused on one point, what do I do with the returned value? I've looked at the 
source code and it appears libxml2 is maintaining a stack of sets of io routines but this seems odd since 
when I start to write something I don't need to specify which set I want to use. This sounds like trouble 
since what if one part of my application starts to write an xml file and before it finishes I register and 
new set somewhere else? Feels like I should be passing this return value of xmlRegisterOutputCallbacks() to 
xmlNewTextWriterFilename() or similar?

- James