opensearch feed calls undefined function

After completing a search and clicking the xml button to get the feed, it reports this error:

Fatal error: Call to undefined function: opensearch_format_rss_channel() in /usr/hm/SPAWAR-4.0/modules/opensearch/opensearch.module on line 209

It looks like the backport from the Drupal 5 branch may have named the needed function format_rss_channel() without the opensearch prefix.