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// set the default feed to atom
add_filter('default_feed','atom_default_feed');
function atom_default_feed() { return 'atom'; }

// remove the rdf and rss 0.92 feeds (nobody ever needs these)
remove_action( 'do_feed_rdf', 'do_feed_rdf', 10, 1 );
remove_action( 'do_feed_rss', 'do_feed_rss', 10, 1 );

// point those feeds at rss 2 (it is backwards compatible with both of them)
add_action( 'do_feed_rdf', 'do_feed_rss2', 10, 1 );
add_action( 'do_feed_rss', 'do_feed_rss2', 10, 1 );