saxon:ptree serialization method

The serialization method saxon:ptree causes the result tree to be output as a PTree: see The PTree file format. This is a binary XML format that occupies roughly the same amount of disk space as standard lexical XML, but which saves time both on serializing and parsing, making it useful if the data has to be sent to another machine for the next stage of processing.

With this method, all serialization properties other than method are ignored.