BOOST_CHECK(ffactory.add_creator(xfid, xfilter_creator));
BOOST_CHECK(ffactory.drop_creator(xfid));
BOOST_CHECK(ffactory.add_creator(xfid, xfilter_creator));
BOOST_CHECK(ffactory.add_creator(yfid, yfilter_creator));
BOOST_CHECK(ffactory.add_creator(xfid, xfilter_creator));
BOOST_CHECK(ffactory.drop_creator(xfid));
BOOST_CHECK(ffactory.add_creator(xfid, xfilter_creator));
BOOST_CHECK(ffactory.add_creator(yfid, yfilter_creator));