code: public static String reduceSurtToAssignmentLevel(String surt) {
          Matcher matcher = TextUtils.getMatcher(
                  getTopmostAssignedSurtPrefixRegex(), surt);
          if (matcher.find()) {
              surt = matcher.group();
          }
          TextUtils.recycleMatcher(matcher);
          return surt;
      }
docs: Truncate SURT to its topmost assigned domain segment; that is,
  the public suffix plus one segment, but as a SURT-ordered prefix.
  
  if the pattern doesn't match, the passed-in SURT is returned.
  
  @param surt SURT to truncate
  @return truncated-to-topmost-assigned SURT prefix