Processes variables for aggregator-summary-items.tpl.php.
aggregator-summary-items.tpl.php
function template_preprocess_aggregator_summary_items(&$variables) {
$variables['title'] = check_plain($variables['source']->title);
$variables['summary_list'] = theme('item_list', array(
'items' => $variables['summary_items'],
));
$variables['source_url'] = $variables['source']->url;
}