#include <libselection.h>
xml_serializer is a typedef name of an aka2::xml_serializer_base derived class.
The xml_serializer is typedef'ed according to the following rules.
Rule1. If Xerces-C++ is enabled, xml_serializer is the typedef name of aka2::xerces_serializer.
Rule2. If babel is enabled and Xerces-C++ is disabled, xml_serializer is the typedef name of aka2::babel_serializer.
Rule3. If both babel and Xerces-C++ are disabled, xml_serizlier is the typedef name of aka2::default_serializer.