Factory method to create a new SAXResult for the given
XMLOutput so that the output of an XSLT transform will go
directly into the XMLOutput that we are given.
Factory method to create a new XMLReader for this tag
so that the input of the XSLT transform comes from
either the xml var, the nested tag or the tag body.
Helper method to get the appropriate xml input source
so that the input of the XSLT transform comes from
either the xml var, the nested tag or the tag body.