Interface for an XML filter.
An XML filter is like an XML reader, except that it obtains its
events from another XML reader rather than a primary source like an
XML document or database. Filters can modify a stream of events as
they pass on to the final application.
The parent reader.
Allows the application to query the parent reader (which may be
another filter). It is generally a bad idea to perform any
operations on the parent reader directly: they should all pass
through this filter.