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// add custom feed content
function add_feed_content($content) {
  if(is_feed()) {
		$content .= '<p>This article is copyright &copy; '.date('Y').'&nbsp;'.bloginfo('name').'</p>';
	}
	return $content;
}
add_filter('the_excerpt_rss', 'add_feed_content');
add_filter('the_content', 'add_feed_content');