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add_action( 'request', array( &$this, 'add_to_feed' ) );

// add to post-feed
function add_to_feed($request) {
	
	if ( isset($request['feed']) && !isset($request['post_type']) ) {
		$request['post_type'] = get_post_types( $args = array(
			'public'          => true,
			'capability_type' => 'post'
		) );
	}
	
	return $request;
}